Why install and run some arbitrary blob that has full access to your computer, when you can open a web page and grant access to a single device?

Yes, and how would you talk to those from a web browser?

Again, the "Print" dialog is not the right way for labels: there is no way of guessing of what it will do to your formatting, or whether it will choose to print A4-sized stuff on labels.

So every website that prints labels should solve that separately?

There is an age-old problem here: how to take content on the screen and produce a printed artifact. It’s not easy, and everyone hates printer drivers, and kernel permissions, etc.

But if you want any portability of content at all, you need something like a print dialog and something like a printer driver. All the WebUSB stuff is just embedding those problems into the application.

Yes, and they run with kernel level permissions.