PDF is a publishing, rather than an editing format. It belongs in the same bucket as .mp3 and .jpg, rather than the .doc and .psd bucket. This is not about how easy or how hard it is to modify a pdf, it's about the intended purpose. The fact that it's meant for publishing means we get to optimise it as such, both in terms of simplicity of the format itself, and in terms of the tools that interact with it. This makes…
The user story here is that PDFs get sent around as forms to be filled out, and that poses a problem for non Mac users or users without sufficient technical skill.
And since you reference mp3 and jpg, you surely know that both formats can be modified in ways that many people will not recognize as modifications. It just pushes the skill level up a bit. But there's always a technically capable person available for hire to modify one of the "permanent" formats you mention.