Its not always just about tabindex. For example it might be about understanding that there are multiple ways to fill out a form that makes sense. Then hiding/showing pieces as needed and/or providing hotkeys to jump from field 1 to field 5 because the user doesn't want to fill out 2,3,4 because they are optional. Its about keystroke optimization. Sure they can press tab 3 times, or they can just press ctrl-5 (or whatever) to get there.

If you watch people use the sabre command line interface (the one from the 1970s?), you can see some of what i'm talking about when people are just filling out the forms with the submission line, its less using the GUI and more just knowing some sequence of keystrokes that results in an action being taken.

AKA its possible to do both, without having the user wear out the tab key, or grabbing the mouse all the time.

I think it depends a lot on how it is used. In the case of sabre, that is somebody's job. They use that interface everyday so the user needs to be able to use the interface efficiently and is willing to learn the shortcuts.

A sign up form that a given user will only use once in their lifetime is another matter. It needs to be simple lest the user abandons the process. Most users in that case are not willing to learn keyboard shortcuts, tabbing over optional fields probably makes more sense here.

It is a problem because with TUI keyboard is the first-class input device whereas with GUI and especially HTML it an afterthought most of times. Yes, there are exceptions like Twitter and Gmail and then there are millions other interfaces where mouse is the only way to navigate.

Modern web culture is very much the problem. The tech can be keyboard-friendly, sure... but in practice, even the most basic stuff, like Enter to submit forms, is often not working because the "Submit" button is not a proper button, but a bunch of JS.