Because PDFs are a nightmare of a format and the only thing that’s is reasonably guaranteed about them is they will render to an image that people can read, the parsing of which will be much less token efficient than the equivalent text
Token economics also are weird. If you design a fancy new frontend that for example uses a cheap model to parse a PDF into text that is fed into an expensive model, you will probably spend more money because you are on API payscale rather than the "max plan" payscale.
For the same reason as why the oil companies want everyone to use large cars.
I'm an engineer and use my coding agent to deal with PDFs all the time. It can reach for unix tools if it needs them.
I don't think I understand why this is a problem - it uses tokens, but it removes drudgery. This is the entire promise of the technology.