I've mostly used cleaned up factory triggers also, but most of my guns have been Remingtons and Winchesters or Rugers, which are pretty easy to do. The one aftermarket custom I did purchase was a Shilen trigger for a Remington. You could get that one scary light and crisp without risk of accidental discharge. Really too light in my opinion. However, I believe the whole point of having a good trigger, is to help shooting from the bench, especially when testing ammo. Of course you won't notice in the field, but in order to control as many variables as possible when testing, you have to have a good trigger. Or at least I do. And there's little doubt in my mind that you can make an aftermarket trigger behave better than a factory one with less work and less risk of accidental discharge. And because most guys don't have the confidence to take a file or stone or polishing wheel to their factory parts, they'd rather just purchase a new one. Also, if you were going to pay a gunsmith to alter your trigger, sometimes the cost difference is negligible compared to buying an aftermarket trigger.

it's not the poundage that concerns me, it's how clean the break is. if you have a spongy trigger, or one with a rough pull, it'll cause you to get errant shots from time to time. the weatherby vanguards are infamous for this, so many shooters choose to just replace the trigger because it doesn't offer enough adjustment without interfering with the safety catch.

some say it was intentionally engineered that way to prevent "backyard gunsmithing"

I assume that people change them for creep and over travel reasons. Some people change them because they want a set trigger (such as a Can-Jar).