MIT is very STEM-centric.
High tech has done very well over the past 40 years. Biotech is doing very well right now in the Boston / Cambridge area.
Massachusetts is a relatively expensive place to live, with relatively high salaries.
MIT brings in a large number of very smart students who work hard. It is quite well known for being academically demanding.
I do wonder to what extent these factors account for relatively high mid career salaries for MIT graduates.