Part of it is because some people can't afford to just "go buy the right rifle" for any particular task, so they will stretch the limits with what they have. Same as someone like me loading way too much firewood in the back of my mid-size pickup and pushing it well beyond its rated payload. I do it because it is what I have, and can't afford to go buy something bigger just for that. Yeah, the next time I go to buy a truck, I will buy something bigger, but that won't be for a while, so I will make due with what I currently have. The only benefit when it comes to cars is that they are easy to rent if you have a rental place local. Guns...not so easy to just rent.
Others, we just like to push the limits. Not sure if you have ever heard the saying about the astronomer and the astronaut. Put simply, one will walk safely on earth, but never get to experience anything more. The astronaut is willing to take the risk to go to space, because that person wants to experience more. For those of us that hot rod any cartridge, we know the risk, and accept it. For us, there is excitement in pushing the limits and seeing what we can actually make something do. Same as when I used to overclock the snot out of processors and graphics cards for computers. I know what they are intended to be able to do, but I want to find out what they are really capable of. Obviously, I am a bit more cautious with firearms since it is my hand/face/neck on the line, but I still want to push them to see what they are capable of.
Having said all that, I will only run it to the ragged edge on guns that I am not too concerned about. My precision AR? Not a chance. My beater poverty pony build? Let's go wild. For the ones I use for hunting/pest control, I want to find the load that most effectively pairs accuracy with performance. Having excellent terminal performance is useless if you can't hit what you're aiming at, just as having something that can knock the nuts off a gnat at 100 yards is useless for hunting if the bullet doesn't give good results on impact.