I clicked the comments here first (bad habit) and presumed there was something more nuanced to the case the OP is trying to make... but you are right, it's quite the WTF.
To be fair I think Atom hides the split commands behind a View->Panes menu and IntelliJ (or PHPStorm at least) has the command in Window->Editor Tabs. (All on OSX). So it's not super clear where the split window functionality is, but it's also not hard to read the docs for your editor..
Edit: Ah I see what the OP is saying now. I'd be surprised if this functionality isn't covered by plugins, though.
To be fair I think Atom hides the split commands behind a View->Panes menu and IntelliJ (or PHPStorm at least) has the command in Window->Editor Tabs. (All on OSX). So it's not super clear where the split window functionality is, but it's also not hard to read the docs for your editor..
Edit: Ah I see what the OP is saying now. I'd be surprised if this functionality isn't covered by plugins, though.