Why? This makes absolutely no sense. It's the same thing as macOS' windows not closing when you close them. It's stupid. They should copy Windows on what makes sense.
I don’t understand the fascination with maximising windows, especially web browsers. What the hell is the point of filling my screen with the window when, for the vast majority of websites, half or more of it is gonna be empty margin?
macOS' windows not closing when you close them
Windows’ apps closing when you close their windows is no less absurd. The same for forcing you to either save or discard documents on quit/reboot.
So run them in full-screen mode. No one is stopping you.
I hardly ever run into a program on macOS that doesn't support full-screen. View → Enter Full Screen, or Control+Option+F.
???
And, as I've pointed out, both behaviors are possible on macOS out of the box.