"How it works" page doesn't explain why domain validation is required every renewal

[mohamedhafez]

At https://letsencrypt.org/how-it-works/, it states that once the domain validation has compeleted and an authorized key pair is established, "requesting, renewing, and revoking certificates is simple—just send certificate management messages and sign them with the authorized key pair"

But if that's the case, how come we need to do domain validation on every renewal, with certbot at least?

Is this just a quirk of certbot, and other acme clients only do the domain validation once?

I'm concerned about the potential for MITM attacks since the domain validation is done over plain HTTP. Or are subsequent domain validations done over HTTPS using the previously issued certificate, thus adhering to the "Trust on First Use" principal?

It would be great if the "How it Works" page could clarify all that!