The traffic never leaves the Tor network. This has a few advantages:

* You don't need a TLS certificate from a public certificate authority, as it is already encrypted end to end

* The exit node cannot attempt to snoop on your traffic (via TLS SNI) or inject content/ads/exploits into your unencrypted traffic

* It reduces load on the exit nodes so they can work on serving traffic to sites that don't have an .onion endpoint

If certificate authorities are a concern, then why do some onion services use https?

Tor public keys don't prove identity (ie, that is cloudflare.com) while such a certificate over, for example, Alt-Svc headers does. Pure .onion certs exist as well and give extra reinsurance that you're on the correct website.

If I had to guess, it’d be because TOR users are not a solid block with identical opinions.

In theory no one can spoof a Tor hidden service. The service name itself encodes the public key, and only the corresponding private key can authenticate. Much better than https, where you have many dozens of "trusted" authorities, any one of which can compromise you.

And also it precludes any attacks a malicious exit node could run on your https traffic, like the other comment says