Yes, engineering has a very structured curriculum in most colleges offering few opportunities for electives if one wants to graduate in 4 years. But I strongly disagree that other curriculums in STEM are that rigid. Math, Biology, Chemistry, and the like – all part of STEM – are in fact traditional liberal arts disciplines. (Many of the great philosophers were also math geeks.) For example, a typical major is ~10 required courses in English and in Math. Attending a major R1 uni and majoring in Math/Science will still allow someone to take plenty of lit/hume electives if one so desires.
The vast majority of kids at a R1 change their majors 2-3 times. Not to mention, that a large R1 has a bunch of other disciplines of which to sample that a LAC does not.
There are a lot of great reasons to choose a LAC, but undecided major is not one of them, IMO.