As you say, some of the parties engaging in the drags absolutely do have the capability. But not all and stopping them has value. More importantly:
A man in the middle attack is _highly_ detectable and will leave irrefutable evidence when detected. So it can only be used secretly if it's used very sparingly. And highly overt interception, if its even tolerated by the public, at least solves the problem of people having no idea (being in denial) they're being watched.
Moreover, because MITM can be defeated by de-trusting the rogue CA or via key pinning using it for "mere" surveillance would destroy a valuable and potent weapon, so they won't do it. It simply isn't suitable for dragnet use.
Its also practically much more costly to scale. (E.g. instead of passive optical taps and cheap packet sampling for targeting they must fully intercept all the traffic and decrypt/reencrypt before they even know if its "interesting") Simply making the watchers have to spend a lot more money per unit of traffic monitored is a win for civil rights because it should result in more conservative use of the capability. Without the crypto the surveillance is maximally cheap and undetectable... anything is an improvement even if it can still be compromised.