Everyone can do generators. Not everyone can loop. The easiest way for the killer to win a game is for them to kill off the weakest link as fast as possible. This is because survivors lose 1/3 - 2/3 of their gen progress once that first survivor is dead (assuming the killer is always keeping at least one survivor busy). So tunneling is heavily encouraged and sometimes even flat out required. The only killers who can get away with not tunneling the vast majority of the time are Nurse and Spirit; because they can make the vast majority of pallets unsafe.
Survivors don't like this; but killers don't have another objective other than to slow the game down as fast as possible and get as many kills as they can.
On the survivor side of things, survivors don't have an objective other than doing generators to escape. And survivors don't have any real form of defense against a good killer other than looping and some mind games. So gen rushing and looping are as heavily encouraged by the game design as tunneling is. Survivors also don't have another objective that they can put a lot of time towards.
The best current solution to tunneling is for maps to be reworked so that they are much less safe for good survivors.
The best current solution to gen rush and looping is for killers to be able to apply more pressure and for looping to be much less safe (see best solution for tunneling).
A mandatory secondary objective is a terrible solution just as much as longer gen timers is; because it would screw over new players too much.
Nerfing tunneling without doing anything else is also a terrible solution just because killers don't have another objective.
I am currently hoping that their upcoming map design philosophy changes will fix some of these issues.