Think of it as a cost and effort threshold. Prevents the dragnet / fishing methods from eavesdropping. It's trivial to force $Company to let you in with letter. The effort to break encryption is not trivial. You have to be doing something wrong to get specific attention.

I mean if you use cert pinning with a public CA you get the same results, aka you can easily spot MITM. I am generally not a big fan of private CAs.