Due to flavor and the power of gaining spell levels
Gaining a new spell level is so powerful for the wizard, that none of their other class features are granted on the levels where they do that, like on 5th. Furthermore, all wizards schools share the levels at which they get their Arcane Tradition features: first on 2nd level, then on 6th. Animate Dead is a third level spell so the earliest opportunity to grant it is on 6th, as the wizard could not use it on 2nd.
Why then grant this signature spell, and nudge players who like to optimize postponing it, instead of granting a more obscure Necromancy spell as a free pickup, like Feign Death?
The answer is flavor. Animating Dead is what necromancers do, so the Necromancy Tradition grants it. Undead Thralls is not alone in this. Every wizard school makes it cheaper to copy school spells to the spellbook, so in effect, every wizard school discincetivizes the wizard to pick their own school's spells when they level, unless they want to lose the cost savings. Why? Again, flavor. It's what they do. The game design puts flavor above optimization here.1
Now, as T.J.L. observes, the main attaction of Undead Thralls is that it makes your undead a lot tougher. That is what matters. Picking up Animate Dead is just gravy.
1 You can see the exact same dynamic play out with the School of Transmutation wizard: they gain their signature spell, Polymporph ony at 10th level. This is a 4th level spell that they otherwise could pick at 7th level, but there is no Arcane Tradition slot at 7th (or 8th, or 9th), and at 6th they are not yet able to cast Polymorph.