There are a few reasons for that.
1. The link to APK cannot be found on the official site[0], so it needs to be looked up in a search engine.
2. Even when downloading from the site, they try to scare you away with a warning [1]. The reason for warning could be avoided by hosting their own F-droid repo, but they refused it, claiming you can download APK and not listening to reason[2].
Though for people using F-droid can still get Signal through the Guardian repository [3]
Thing about the signal APK and the Guardian one is that, it still have the so called "crap" in the final APK, it just runs a background service when required google services are not detected, causing battery drain for many[4].
The drain could also be avoided by supporting UnifiedPush (it can fall back to FCM when it's detected), but they don't want to do that either[5].
[0] https://signal.org/download/
[1] https://signal.org/android/apk/
[2] https://community.signalusers.org/t/how-to-get-signal-apks-o...
[3] https://guardianproject.info/fdroid/
[4] https://github.com/signalapp/Signal-Android/issues/9729
[5] https://community.signalusers.org/t/use-gcm-fcm-alternatives...
I walked up and introduced myself and said that I was a big fan, appreciated his hard work, etc. He looked at me coldly and just said "so are you going to buy something?" and motioned toward the booklets. I didn't need a printed copy of the `sed` man page so I shrugged and he seemed quite annoyed, turned to his assistant with a notebook computer and started dictating something to them, as almost to make it clear that our interaction was over.
I'm not sure what the point of posting this is, but that's my RMS story - it was my first "never meet your heroes" moment, I guess.
But, RMS is known to be socially awkward, the same goes for many autistic individuals. It's just that he doesn't mask and comes out as “rude”. If send an e-mail, he will usually take his time to write down a succinct response.