Because window managers. Windows 7 and before definitely didn't make it obvious how to make a window half a screen, Windows 10 makes it a bit more obvious I think, but you're still limited to 2 windows side by side. Ubuntu's default window manager has the same behaviour as W7 and Lubuntu's doesn't have keybindings for this by default at all.

You can of course resize windows manually but it's tedious compared to a 2 button shortcut, especially having to hunt the 3 pixel wide resize button with wrist issues.