I've asked our TPA about this before, and the answer I got was this (paraphrased):
"The total contribution limits, including deferral and matching, used to be much lower. So in order to prevent people from going over this, many companies put percentage caps on your contributions just to avoid figuring out the numbers if someone got close. Several years ago, the caps were lifted and the rules changed such that there is NO limit to what percentage an employee contributes, provided that 1) they don't exceed either deferral or total contribution limits, 2) they do not violate 401(k) rules."
Item 2 is important, since it is the only practical reason for companies to still cap your contribution. That's the somewhat lesser known rule that limits highly compensated employees (HCEs) from contributing a higher percentage to the plan than the other employees at large. This is a legal test your plan must pass yearly, unless you are under something like a Safe Harbor plan.
Anyway, the point was the caps are generally there to make the life of your HR department easier. If they have an HCE cap of 20%, it's easier to just cap everyone at that amount. Or, even if there is no need to cap, many do it just because they always have and are too lazy to change it.
When we set up our plan, I pushed for no cap, and we agreed finally on an 80% limit. Even though I didn't need it that high, I wanted the other employees to have the flexibility of hitting the cap even if they had a lower compensation package.