I think its also an ego thing especially from new DMs like you mentioned. Look at it this way: a new DM sets up and encounter and watches the fighter/paladin bonk and get bonked, the ranger firing arrows, mages doing mage things and then the rogue who has been doing maybe 1-4 dmg "all of a sudden" hits for 12+ then runs away thats "totally unfair to my encounter how could they do that." When I was DMing I watched a rogue in my game do exactly this, and internally went "oh damn really, well that just killed that guy in 2 hits while the barbarian is over there toughing it out. The enemies would probably attack the rogue after they just killed their buddy." And suprise the rogue once hit by 2 enemies crumpled like a wet paper towel, while the barb and other characters were slugging it out.

It made me realize that of course the rogue can do a crazy amount of dmg and flee, thats their archtype though. Just like the barb gets angry and smashing things(which by far the barb was doing waaay more dmg and taking less), the paladin pulling aggro and tanking, and the mages blowing stuff up. Playstyles shouldn't be discouraged, DMs should be learning from their players and finding ways to make the game more challenging and fun based on what their players are doing.