Hello,
The reason we don’t provide standalone APKs is that normally there is no straightforward and automatable update mechanism. That said, Jami is already in the F-Droid.org repository (https://f-droid.org/en/packages/cx.ring/) so you can install F-Droid and then install and update Jami via F-Droid. We are also looking into setting up our own F-Droid repo(s) on https://dl.jami.net as well, and will announce it on this forum when ready.
Hope this helps 