No it isn't. You can still install and even build packages using the manifest separately. It is just convenience, but doesn't prevent you from installing them yourself.

Prevent, no. However making your promoted paradigm of software installation rely on repos and package maintainers has lead to practices that make it unlikely that you will find any given piece of software in a form you can just drop onto your system and have it run. If you're really lucky there's a static version or an AppImage, if you're unlucky you're pulling down a docker container of someone's build environment and compiling it yourself, if you're really unlucky you have to do without the docker container.

That is on software developers, many of which refuse to package their software. There are Snaps, Appimages, Flatpaks, etc. Free software isn't like Windows where developers are always packaging their software. Package managers don't impact this whatsoever, and what you're basically complaining about is that unpaid contributors make something easier for people that they wouldn't otherwise be able to easily do without them.

I'm complaining that the community's reliance on package management has set the expectation that unpaid contributors make software work on the platform.