Probably money
But seriously tho, a good question. On one hand there is the cost issue - A practical application of volume pot after the output will result in high output impedance and hence require another buffer stage to bring the impedance back down. But cost doesn't explain the whole story and something that is needed will need to be added regardless of cost.
Which brings us to the second point - The current meta is that line-level output connections do not require a physical volume control. Like the 3.5mm out from your computer, or your phone, or the RCA from your BD player, and DAC. CD players / walkmen used to have a physical volume control, but my MP3 player from 2002 has switched to digital. And since most audio hosts are already capable of digital volume control on their own and the DAC just needs to do one thing, the addition of another volume control is deemed unnecessary by the meta especially in a use case where most equipment are expected to have a dBFS between 300mV to 2V.
Headphone out on the other hand still warrants a volume control be it digital or analogue, since nobody knows what output level is really needed. Same for speaker amps in a similar vein. And going back to the BD player example, we can see that the necessary volume control is one attached to the output equipment, or something that is your main control hub (e.g. TV, or AVR), while the rest of the chain... not so much. You can always buy a DAC with a volume control and multiple inputs so it becomes your main control hub... which reminds me, we call integrated amps integrated because they come with a volume control with input switching, while power amps don't. So ultimately, the meta decides everything.