Great question. This is difficult to answer in a satisfying scientific way. Generally, in science, we ask questions that are testable--that is, we should be able to verify (or debunk) our hypothesis with an experiment. So the best that science can say here is: we think the speed of light is the absolute speed limit because we've never observed anything traveling faster.

And we've tried many times, for example in particle accelerators such as the famous Large Hadron Collider (LHC). All the experiments we've done seem to be consistent with Einstein's theory of special relativity (and his later expanded theory of general relativity).

It turns out that when you give particles more kinetic energy near the speed of light, you end up increasing their relativistic mass instead of their speed.

Fundamental physics (or arguably science in general) is often a huge guessing game. We come up with a model or a theory and assume that it is true. Then we run experiments that either help us build on our model, or invalidate it and force us to start over. These theories are usually impossible to prove, only possible to disprove. If we can't disprove a theory, and it can explain things that nothing else can explain, that theory has scientific value.