Why are good USB/SPDIF adapters neccessary, why not built in DACs?

[dodap]

In the last few years a lot of products came into existence that attempted to improve the signal quality coming from computer USB outputs: Galvanic isolation, converters into SPDIF signals, exchange for the computer 5V power for the converters, even better USB cables. It is obvious that these additions to an existing rig can improve sound quality. 
 
Bur why are these improvements not built into better DACs as this probably would be not only space savin but cheaper than all those seperate units?

 

[padam]

More and more DACs like those will come to the market, although they either need to design their own custom solution or license the technology from someone else, neither solution is cheap so in the 'value' category they either need to save on this USB solution or the DAC itself (and unsuprisingly in most products they save on both), and and no matter what they do, they compromise the sound.

 

These converters give better flexibility to connect them to a DAC where it didn't worth for the manufacture to design it with a proper USB input but the DAC part was designed 'seriously' (although this is subjective).