Yeah, because there's boob physics and then there's boob physics. As everything else gets more realistic, static breasts look more awkward, like the character is made of plastic, if they don't bounce a little.

Its pretty noticeable in anime when the big titty waifu runs down a flight of stairs without even a little bounce at the end. Of course that's in 2d where it's less realistic, and the bounce has to be animated by hand. Leaving it out is much more noticeable in 3d, and implementing such basic physics interaction is trivial in any modern engine. (Rigging and modeling might need to account for it, but it's become easier from that end too.)

There's a big difference between gainaxing with each breast having its own physics calculation and taking over a full second to come to rest, and a character's breasts taking just a split second to catch up to a change in velocity. The first looks silly and sexualized (not that there's anything wrong with that in the right situation) while the second is basically needed for an animation to feel satisfying at all if the character is sufficiently realistic and has a medium sized or larger bust.