So there's a bunch of stuff there.
1 - Why not use regular lines?
A) You don't always have all the best talent on your top 2 lines (think Kessel on HBK, Horny this season, Guentzel's brief stint at 3C, etc.etc.) B) Lines change pretty often, particularly in Sully world, but the PP unit can stay pretty consistent, so it results in better chemistry C) PPs always get better results with 4 forwards, so you've got to scramble lines anyway, so why not go all the way D) these guys have egos and want to play with the best. Not playing PP with each other would be unpopular.
2 - Why always the drop pass?
This article explains the mechanics of PPs better than I can but the basic reason is it's a response to the most common 1-3 PK forecheck. The first man up is trying to funnel the puck carrier to one side or the other, so the centre man in the three can converge, and you've got a 3 on however many in a tight spot. It's pretty difficult to gain the blueline like that. So the idea behind the drop pass is the puck carrier forces the first man to bite, then makes the safe pass behind him, and the two trailing forwards can initiate an attack on the side without the first forechecker and get a 2 on 1 against a stationary dman to force the zone. The issue is you'll see teams make the drop pass without really forcing the bite.
Incidentally, I feel like I occasionally see the Pens sort of do a 1-1-2, with the first forward pretty high and forcing the dump-off early, leaving the second forward with a chance to funnel the puck carrier as well.
3 - Why no radical new approach?
I know you didn't quite ask this but it's kind of there and the issue is time. And conservatism too, but mainly time. How long is Pens training every day? Couple of hours? In that time, they're breaking down the last game, prepping for the next one, giving new linemates a chance to know each other due to chemistry... they don't have the time to implement new systems to the point where it won't lose the next few games. Maybe they would at a camp like this one - in fact, a camp like this is the perfect opportunity, and maybe we see some innovation in hockey - but mainly it's just hard.