Because genitive plural ending -ов is only used with masculine nouns whose stems end with a hard consonant. The singular nominative form from which детей derives is not дет. The singular forms of this word have fallen out of use in prehistoric times, but if they hadn't, nominative singular would be *деть, and it would be a feminine word. So it declines not like цвет or атлет, but like четверть or лошадь, and such words have the gen. pl. ending -ей.
Not quite. Historically speaking, дитя harks back to Proto-Slavic *dětę (Old Russian дѣтѧ), a form with a suffix denoting younglings. There were more words like this; we have attested examples from Old Russian and Church Slavonic for ос(ь)лѧ = “young donkey”, ж(е)ребѧ = “foal”, телѧ = “calf”, ꙗгнѧ = “lamb”, etc. They had a peculiar declension pattern, with vestigial /t/ in all cases and numbers except singular nominative/accusative (and vocative). For most words, this declension survived in Modern Russian in the plural: we say ослята, жеребята, телята, ягнята, etc. It didn't survive for дѣтѧ in the plural in standard Russian, though it did in dialects and in standard Ukrainian (дитята); Polish also has its cognate (sg. dziecię, pl. dziecięta), a bookish word. In the singular, we still have дитяти, дитятей, though they're mostly used jokingly in modern language.
That is, the word survived in unsuffixed forms in the plural, as дети (< OR дѣти < PSl děti) etc., but was displaced by suffixed forms in the singular. It's a kind of suppletion, though preserving the root. We see a very similar phenomenon with words for animal young. Their singular forms feature the additional diminutive suffix -(о)к, before which the old nasal vowel -ѧ- has mutated into consonantic -н- (телёнок, телёнка, etc.); plural forms were not suffixed with -(о)к and preserve the old declension.
(Not OP) I usually do that with the irregular words or exceptions, but the curious part of me sometimes like to find out if there’s a reason that they are the way they are. And because Russian is so rules based, usually there WAS a rule or reason something is done a certain way… and that fascinated me. Even if that information does nothing for my learning of the language.
The singular form of the word is actually дитя. Дитя -> детей is irregular. But nobody uses дитя anymore: it has been suppleted by the form ребёнок in the singular.
Дитя -> дети is also suppletive. The singular of дети would've been *деть (survived in the language as детка, деть + suffix -ка). The plural form of дитя would've been *дитята/детята
because the word дети has a soft stem (и is a softening letter, like я/ю/ё/е and ь)
the same with the word море, for ex:
море --> моря --> морей
Add. I'm specifically giving you a neuter word as an example, because "дитя" is a neuter word, this can also make sense, as well as animation
Add.2. дитя in general, it has specific declension system, which is quite unusual for untrained person, so in this case you just should remember
because the t in the stem is palatalized
Because genitive plural ending -ов is only used with masculine nouns whose stems end with a hard consonant. The singular nominative form from which детей derives is not дет. The singular forms of this word have fallen out of use in prehistoric times, but if they hadn't, nominative singular would be *деть, and it would be a feminine word. So it declines not like цвет or атлет, but like четверть or лошадь, and such words have the gen. pl. ending -ей.
Singular, still usable form of дети is дитя, not деть. Деть is an ironic, intentionally incorrect form like человеки или людь.
Although there's antient slavic дѣть and дѣти if you mean that.
Not quite. Historically speaking, дитя harks back to Proto-Slavic *dětę (Old Russian дѣтѧ), a form with a suffix denoting younglings. There were more words like this; we have attested examples from Old Russian and Church Slavonic for ос(ь)лѧ = “young donkey”, ж(е)ребѧ = “foal”, телѧ = “calf”, ꙗгнѧ = “lamb”, etc. They had a peculiar declension pattern, with vestigial /t/ in all cases and numbers except singular nominative/accusative (and vocative). For most words, this declension survived in Modern Russian in the plural: we say ослята, жеребята, телята, ягнята, etc. It didn't survive for дѣтѧ in the plural in standard Russian, though it did in dialects and in standard Ukrainian (дитята); Polish also has its cognate (sg. dziecię, pl. dziecięta), a bookish word. In the singular, we still have дитяти, дитятей, though they're mostly used jokingly in modern language.
That is, the word survived in unsuffixed forms in the plural, as дети (< OR дѣти < PSl děti) etc., but was displaced by suffixed forms in the singular. It's a kind of suppletion, though preserving the root. We see a very similar phenomenon with words for animal young. Their singular forms feature the additional diminutive suffix -(о)к, before which the old nasal vowel -ѧ- has mutated into consonantic -н- (телёнок, телёнка, etc.); plural forms were not suffixed with -(о)к and preserve the old declension.
It's not a given that you'll find a definitive answer; just accept that this will be correct.
(Not OP) I usually do that with the irregular words or exceptions, but the curious part of me sometimes like to find out if there’s a reason that they are the way they are. And because Russian is so rules based, usually there WAS a rule or reason something is done a certain way… and that fascinated me. Even if that information does nothing for my learning of the language.
It’s irregular. Every language has irregular forms that just have to be learned like this.
Another similar irregular is люди -> людей
It's really not irregular. As Stock Soup noted, nouns with stems ending in a soft consonant regularly form their plural in -ей.
The singular form of the word is actually дитя. Дитя -> детей is irregular. But nobody uses дитя anymore: it has been suppleted by the form ребёнок in the singular.
Дитя -> дети is also suppletive. The singular of дети would've been *деть (survived in the language as детка, деть + suffix -ка). The plural form of дитя would've been *дитята/детята