these days, if a software doesn't use the design language of a browser, users think it's old and busted

The default Windows taskbar doesn't haven't given you tabs for a very long time, has been mostly unusable if you use it as tabs recently, and only has 1 level of tabs, while keeping tabs inside the application gives you another.

That said, I'm not sure notepad benefits from it.

if we were still allowed to have a vertical taksbar we could have enough tabs indeed

To answer seriously, because workflow is a tree. And the outer levels need task-specificity. Browser tabs need different display options and available actions from text editor tabs, etc.

A generic taskbar (Windows one included) is only passable for the root level of switching contexts (on *nix there's also couple levels below that: virtual consoles and virtual desktops of the DE).

always the first thing i disable in Notepad++

Oddly remembers tabs for me just fine.

You can disable it. Settings > When Notepad starts > Start new session and discard unsaved changes