Wasn't PDF supposed to be a read-only, print-only medium? You're supposed to edit the source material (like a doc) and export that to PDF, not edit PDF directly.
That works if you have access to the source material, which you don't often get. Most of the time it will suffice to just draw over it using Xournal++ or simmilar, but there is always that odd case where you need to change what is already in the PDF
Well, there's Acrobat. I think that the issue is that PDF is a deceptively complicated format. It's basically a programming language for describing pages to print. So it's not like parsing a JPG or something, you need a PDF interpreter. And I imagine the Adobe one is encumbered.