Among many other reasons, because such entities repeatedly try to weaken Internet security standards and software to accommodate their expectations. That's an attack, and deserves an appropriate response.
Also, because today, everyone doesn't always have enough choices of potential employers that they can avoid such policies.
Also, because nobody should ever get used to the idea that their network is MITMing their traffic, not least of which because that normalizes such technologies on other networks and in other contexts.
Also, surveillance technologies like this will be abused by people with power over others; there's a long history of such abuse.
And those are just the reasons off the top of my head.
> Go ahead, you're increasingly unlikely to win that battle.
What happens when libraries and software starts dropping support for old, insecure protocols, and the new protocols are designed to treat MITM as an attack? What happens when, even before that, companies that MITM have to replace cheap MITM infrastructure with incredibly expensive MITM infrastructure? What happens when software and services start treating the resulting decreasing percentage of companies that try to MITM the way they treat companies still running IE6, and tell them "you'll have to upgrade your network"? What happens when people trying to MITM find they can't run modern software stacks, and their auditors complain that they're not upgrading? What happens when they're spending huge amounts of money maintaining forks and patches? What happens when the engineers who would have to maintain those patches don't want to work on a hostile network, and the ones that remain are more expensive? There are many things people can do to make life difficult for anyone trying to MITM.