Many airports (in the US, at least) make much more money from parking fees than everything else. On top of that, at least some operate their retail in a royalties model too (ie 20% of all sales goes to the airport, before any costs).
Thus, there is very little incentive to keep you in the airport less, and multiple incentives to keep you there longer.
That's what makes the 2-hour early 'rule' so egregious. Manipulating passengers to sit around in a Disneyland-priced mall because of the 1 in 5 chance the TSA screwed up their staffing that day.
I think Singapore has a pretty good experience worked out now, the immigration is automated just with an iris scan.
Additionally moving security to the gate rather than having a single security point for all flights also makes the wait here shorter and more importantly predictable.
If I'm just taking a cabin bag I always arrive 35-40 minutes before a flight and have had no trouble with time, normally enough time to grab a coffee too before boarding.
Thus, there is very little incentive to keep you in the airport less, and multiple incentives to keep you there longer.