Sooner or later, almost everybody pulls their low back, and sometimes, to the point of permanent, disabling injury.
If you are more inclined to a creationist view -- which, by the by, is not incompatible with evolution, as long as you look at it the right way -- why God gave humans weak backs is beyond me. Ask Him.
Um. Anyway, the torso, the trunk around which we are built, can be strengthened. There are exercises to make your back muscles stronger, and while even the most powerful of them can still be strained, a strong back is better than not.
You can also compensate for the intrinsic weakness by developing the other trunk muscles, notably those in the front and sides, the rectus and transverse abdominis and obliques, like that. A strong belly will help you keep from hurting your back if you learn how to do it.
I don't think sit-up or crunches are the best way, but better than nothing. I like doing chins in an L-sit. And there are some others I like, including an old yoga exercise called rectus rolling. Hard to see this if your body fat is too high -- I think mine is probably a hair under 12% or so now, higher than I want -- I'd be happier at 9% or 10% -- but I can show this one even so.
Observe:
The simple way you do this is to lean forward and press against your thighs with your hands. Suck your belly in and lean onto one hand, then switch the pressure to the other hand, and voila! you get the roll. Experts can do this without using their hands, but I'm not there yet.