I've had plenty of pushback from managers (and customers) who didn't have MBAs. The difference is usually between how long things "should" take everything going well and how long things often do take.

On a small/personal scale, you can (and I do) sandbag to a certain degree. But it's hard on a large project when everyone and their brother is looking to trim any fat out of the estimates.

And how do they know how long anything should take, especially when they don't understand the details of the process? Sometimes "trimming fat" just isn't realistic or even possible

All that matters is that you get the contract. Sunk cost fallacy keeps your customers. It probably also helps that everyone operates like this, so its typical to expect things to come in late and overbudget to a degree.

The counterargument on large projects is there's so much coordination between different groups that the location of the "fat" becomes opaque and difficult to identify.