The answer to your 2nd question might be Google's custom silicon: https://blog.google/products/pixel/tensor-g5-pixel-10/

The answer to your first question may simply be they want to sell more Pixel 10 phones.

The investment into custom silicon is more likely to pay off when new and exiting features are exclusive to the newer platform.

That hardware is completely unrelated to such a simple feature. Something like AirDrop will only use fairly trivial crypto, which most likely ciphers with full acceleration available but even without it would work fine with plenty of performance headroom.

Neither Apple nor Google is doing anything revolutionary with their silicon for such a standard compute task. It's really mostly minor tuning to get a more optimal part instead of an off-the-shelf chip catering to other uses too, with die area and power consumption "wasted" in your setup.

previous pixel phones also had custom Google silicon, just with some Samsung IP