My first few DACs clearly had a worse implementation of USB than the other inputs. It wasn’t until my current DAC, which I’ve had for about 4 years now, that I felt all inputs, including usb, were equivalent.
The OP seems to assume that usb is universally regarded as the best sounding input. Besides my personal experience, simply being a member here and reading the comments through the years has led me to believe otherwise.
Usb is the most ubiquitous output. Every computer has one as do multiple consumer electronics that one may wish to plug into a DAC. Therefore DAC manufacturers have to add the add the input. The need for reclocking makes it more difficult to work with, but they have been at it for a few decades now. Some manufacturers may actually prefer to work with usb. More power to them