Short answer: Bookshelves are a lot sexier than document libraries.
Long answer: If I really like a creator's work, I will usually support them monetarily, whether it's through Patreon, buying their merch, or anything else that might help them out. I've got a little bookshelf filled with Tapastic goodies, from prints to bookmarks to printed books, even a few custom doodles from the creators themselves. I love everything I've received from my fellow creators (some of them were purchased, but there are others that weren't, like raffle prizes and supporter goodies, and even a couple gifts). There's something a lot more personal in owning a physical copy of someone's work. Next to the fact that most comic artists only sell limited runs of their books (it's easy for hundreds of thousands of people to own digital copies, but having a physical copy usually isn't as easy or common) there's just that special feeling in holding a physical copy of your own, or someone else's work.
Aside from all that, I find I work a lot better when I work to panels on a specific size page. When a canvas is "infinite", I feel like I'm not making a dent in my work, and that I've still got a lot to go, and that can be a little hard on my motivation. Along with that, despite the fact that vertical canvases are basically "unlimited", most vertical comics still look the same :I They're usually just panels scattered down a page, very few of them actually getting creative with their space.
It shouldn't matter what your canvas size is. It's all about perspective. You may consider a single 8.5 x 11 page a "restriction", but others may consider it a perfect fit, like wearing the perfect pair of slippers that snug their feet just right. It's up to the creator to come up with creative ways to panel and layout their work - no amount of vertical space will make a comic look any better or be any less restricted if the creator isn't willing to think outside the box, and this goes for print creators too. There are restrictions with vertical format as well - namely, the width size. Unless you can get creative about it or you plan on just telling your readers to rotate their phones, you can't do landscape/horizontal spreads that deserve every inch of space they need to show off an epic scene that requires a horizontal plane vs. a vertical one. We live in a "flat" world - so it makes sense that bigger shots are typically done horizontally, and that's where vertical, digital comics have their flaws.
But I think I went off topic lol
Back on the topic of printed books, even if I only ever print copies for myself, having a printed copy is my choice, and no matter what your opinion is on it, if they want to spend the money on getting books printed, that's up to them. Most people aren't going into comics expecting riches, and the ones that are printing books are ones that know they have the readership willing to buy copies. Whether their buyers are collectors, die-hard fans, or just want to throw a little money the creator's way, what the creator do is up to them.
Printed media is not dead yet, so don't treat it like it is.
Plus anyone can do a digital comic these days - they just need to know how to make JPG's and upload them to the Internet. Having a physical copy is not something everyone has or can have. A lot more effort is required to make a physical book, and the payoff is so worth it if you ask me (whether you're selling them or just printing them for yourself).