In order to preserve the sacrosanct simplicity of naive shell scripts. Seems like a very noble goal to me.
The only unexpexted compexity arises when you want to deal with filenames having mixed spaces and nbsps. But I'd say that people who do that had it coming.
The filesystem is way more important than /bin/sh and and any complexity added there will trickle down to all programs, not just shell scripts.
It's not worth adding hacks on the FS to patch defects in poorly written shell scripts (which are being replaced en masse with python/nodejs/even weirder yaml files/systemd units/etc... anyways)
To mishandle spaces you have to split an input w/ filenames by whitespace, which is not that common of an operation outside of a shell.
If you actually test this, you'll realize a ton of Windows programs get it wrong.
Also, in general this is a poor argument. The goal of Linux isn't to be as much like Windows as possible, because Windows sucks ass. Nobody in their right mind would use Linux if it was just Windows but, presumably, shittier. The entire appeal of Linux is that it isn't Windows, and it isn't MacOS.
Even zsh has fixed this. It's just /bin/sh and bash that are annoying.