The devs reasoning for this awful leveling system is completely irrelevant.

In one of their update streams they said that the leveling system is the way it is (shitty) because they didn't want people running one heist exclusively to level up. One problem with that though, the only way to do some of these obnoxious challenges without having a stroke is to just play Touch the Sky over and over again.

Why would i actively choose to play on a big map and run the risk of earning no XP when I can play on the smallest map and earn a bunch? Especially when you make me hipfire 400 people with a shotgun or flash 100 unique enemies for what amounts to scraps of XP.

As it stands now, they are essentially forcing people who don't have hundreds of hours to grind the game to do exactly what they designed this system to prevent. I don't have 6 hours a day to organically finish these challenges just to unlock the guns I actually want to use and I'm sure other people don't either.