Tor is already encrypted, that’s why you don’t need TLS. Some services (Like the hidden service from Facebook back in the days) have https but that was more of a vanity from what I remember.
It has a functional difference as well, lots of new client-side features (like webcrypto) only work on "Secure Origins" which .onion isn't, but websites behind TLS are. So if you wanna deploy say something that encrypts/decrypts something client-side on .onion, you unfortunately need TLS today otherwise the APIs aren't available.
Of course browsers could fix this, but I don't think they have any incentives to do so. I guess Tor Browser could in fact fix this, and maybe they already do, but it'd be a patch on top of Firefox I think, something they probably want to do less off, not more.
One could argue, given the limited bandwidth of the Tor network, that by using it when you don't need it, you make the experience for those that do need it worse (looking at you everyone who tries to torrent over tor).
edit: oh, is the last relay the onion service? So the entire chain is encrypted?
https://proton.me/blog/tor-encrypted-email
In the above blog post, they seem to imply that they made HTTPS mandatory for Proton Mail over Tor for security reasons.
tl;dr: Pressure from browsers, enterprise, and the overall ecosystem to use HTTPS (e.g., unavailability of advanced web features without HTTPS) is pushing for the use of HTTPS without exception, even for .onion sites with no significant technical advantage.