Because it’s free hosting, and if you use it for code, you already have your project on there.

Part of the issue is that doing anything more involved requires the developer to now put work into setting up another solution, and usually the developer wants to work on the project, not on making a website or launcher for the project.

Also worth noting is the typical Linux ideology, where the developer isn’t in charge of getting software to users, they’re expected to set the project up so it compiles consistently, and the distribution maintainers are then expected to package it for their distro.