I'd imagine for the same reason corpsefolk aren't playable. Devs came up with an idea for a race, decided for whatever reason it couldn't be playable, and instead of delivering a pruned down version of it to be playable like they've done with a few different races, instead made something different that fulfills the niche as an alternative and now don't want make redundancies.
Being more generous I'd speculate that anacites were perhaps deemed too variable, but that's pure speculation on my part.
No, an SRO is a different thing.
"SROs are most common on Aballon, though they are different from the native anacites"(pact worlds)
Okay So.. the Anacites have Light Dependency which is a strong weakness for any PC to have they also have Full Construct Immunities which are overpowered. The SRO however, is playable because it lacks these two traits.
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"Why aren't mammals a playable race" is also a bit overly broad. Anacites have a lot of subspecies.
That said, as I understand it anacites probably don't have souls, which is still something of an big deal. SROs and androids are apparently special in that they're constructs that do have souls.
But more mechanically, you could of course make an anacite playable race, but to keep it balanced, you'd have to pare down a lot of their abilities (especially construct immunities) to put them at the same level as other playable races.
At which point you're probably not that different from an SRO, maybe even weaker (since SROs have unusually many immunities, maybe more than a playable race should have compared to other playable races).