The reason is just a matter of priorities. AppSheet, before it was sold to Google, implemented the PDF generation using a 3rd party vendor. When Google took over, revamping the PDF Generation just hasn’t been at the top of the priority list. To move it up there needs to be some indication of interenst. Yours is a good start.
To gain more traction and interest in this, I would to see if there is an open Feature Idea and post on it. If not, then open a new one.
FYI, there has been a series of Tip/Trick posts by @SkrOYC that detail a different way of producing PDF’s. I haven’t ventured into it yet but I understand it uses tools that give you a LOT more control over the result. See the series below
Part 1 - Almost pixel perfect pdf reports from automation
Part 2: Pixel perfect paginated reports
Part 3: Pixel perfect reports with Skia (AppSheet’s PDF generator)