There's not reason you can't have a "dedicated page" with a back button (or even respecting the real back button) in an SPA. Modals are created by lazy designers, devs only implement them.
Canvas in particular has issues web devs have to deal with beyond their "normal" duties. Accessibility is simply not even a thing in canvas. Google Docs had to implement an entire parallel invisible DOM structure to emulate what they draw on the canvas when they were switching over to the canvas. And, of course, that DOM structure has to be in sync with canvas.
Especially when the modal covers half of the page, with a form with dozens fields.