32 bit DACs or 32 bit ADCs would just add to the expense and provide no real world benefit. Even with 24 bit DACs or ADCs, the least significant bit (LSB) is below the noise floor. So is the next LSB and more. While properly implimented 24 bit DAC and ADCs can give a very worthwhile improvement over 16 bit, it is far short of the 8 bit difference.

32 bit digital processing, on the otherhand, is worthwhile compared to 24 bit processing. 32 bits gives you a lot of leeway for digital junk (truncation, roundoff errors and such) that accumulates due to every digital operation, the total error part of the digital word working it's way from the LSB end of the digital work towards the MSB of the digital word as the digital processing continues. 32 bit processing allows more bits to be "wasted" at the LSB end (for a given amount of proccessing) and still have enough "untained" bits to feed the 24 bit DAC.