And the bits are basically free. If we had very cheap 24-bit floats and nothing bigger, maybe we'd use those, but we've got cheap 32-bit floats, so those are fine.

The most important property of floating point is "infinite headroom". In integer space, sixteen times quieter means 4 fewer bits of audio, get the levels wrong badly enough and people can hear your mistake even if you fix it later - but in float space it barely makes any difference, so long as the levels are correct in the final consumed audio nobody cares.