There's always https://f-droid.org - it has somewhat vetted free-as-in-freedom apps that must compile from source.
I think developers don't put APKs anywhere else because they think they won't have an audience for those anyway. But it's a pity anyway. For example Humble Bundle packs with Android games were quite successful and the distribution method was through download links.
However, I would like to point out that the Humble Bundle packs, while available via download links, also is available for download using a Humble Bundle app that acts like a package manager. I don't know how much that would change things, success-wise, but I thought it was worth noting.