> it's not "apt install" but it's pretty close

It's not close, though, it's very far. With apt, I know things are going to go in their proper place, and are going to get cleaned up afterwards (not to mention the biggest benefit, automatic updates).

With a shell script, I don't know what sort of stuff it puts where on my filesystem, and I can't automate the installation easily. My preferred order of installation procedures is: "apt", "manual", and "random shell script" is a distant last, to the point where I sometimes won't try software if the installation procedure isn't sane.