Telegram is indeed neither private nor secure (that is if you don't use secret chats), but the stupidity of people believing the narrative of

Russia-controlled servers

will never cease to fascinate me.

You know which group of people know for certain that this is not true?

Russians.

How can they be so sure?

Because of their government's constant attempts/threats to block it / slow it down. All in an effort to make people move to an actual messaging app controlled by them (MAX messenger).

Yes, absolutely, Russian government is trying to stop people from using an app they control, this totally makes sense.
And if it doesn't, you can always come up with some totally reasonable and totally not a conspiracy theory of them just pretending to trying to block it so that you think it is secure. And people might even believe it. Except for Russians, for whom Telegram voice calls are broken (blocked) again, and MAX is advertised as an alternative.

For fuck's sake...

It's a social media platform, not a secure messaging platform.

it's user friendly, used by many normies, better than some alternatives (whatsapp, etc), advertised as private, etc. at least until recently they also mostly ignored governments' request for data, so it kinda was private for criminals as well.

Telegram is less policed than the American apps with backdoors. It's popular with many people making it easy to connect. (It has a lot of good features). The options you listed all have adoptjon challenges for non-technical people.

I guess that you only see the tip of the iceberg in these "darknet forums". They probably favor visibility for the end-user instead of security, due to their target audience. XMPP and GPG is pretty popular on DNMs.

Over a billion users, and very popular in old eastern bloc countries already.

Yes central control, and controlled by people who don't have the most robust encryption or opsec (CEO house arrest in France for example)... But over time they have demonstrated they don't care too much about most low level crimes.

So it's in the sweet spot of good enough and convenient but not great and not perfect

I often have the same questions about bitcoin

Because it is secure, that is why.