How many users compile Firefox or LibreOffice themselves, rather than just downloading the binaries? And don’t even most users of popular package managers just install binary packages?

Installation is not the same as distribution. All upstream packages that I am aware of distribute in source form.

Yes, and if you want to take this route you can either maintain your package yourself on every single major distro or deal with support tickets for issues that have either already been fixed in old versions or arise from strange interactions with dynamic libs.

If you want to draw the boundary that way, fine. But to me, the point of open source is that I can get the source code and compile it myself, if I want to and have the necessary resources to do so.