This is the somewhat depressing but accurate answer. HTTPS doesn't mean you are communicating sensitive or private data, it means you want the client to see what you send them, and not something else.

To be fair, triviality is relevant to the tooling you run and there are many downsides - extra negotiation and traffic, unsupported on older clients without using up an IPv4 address, certificate expiry, etc.

For most cases these are non issues, but there are many scenarios where those things can outweigh the potential of your ISP modifying/reading. If that's still a concern, you can tunnel through your ISP to a more trusted exit point.