You could dig deeper into this and probably find a very complicated explanation, however there is a simple reason: Nobody knows why.
Yes, coloumb's law was empirical and it was observed how the charges behaved and then the law was derived. Most laws in classical physics are empirical and were observed at some point of time by really smart scientists and put into theory. Even the formula for the wavelength emitted for transition of an electron in an Hydrogen atom was discovered by trying to relate the numbers that were seen when the hydrogen spectrum was observed. This was accomplished by Balmer who was a teacher. Get used to this fact. If we could derive everything from scratch, why would there still be unanswered questions? If this answer was helpful, take some time off to accept it and if you require any further information, do not hesitate to comment!
Edit: Before you say that this isn't how stuff works and we can explain using this or that, remember that my point here is to tell that every theoretical concept requires some observational backing or assumption. Nothing can be derived from scratch. Nobel Prize Laureate for Physics Richard Peter Feynman has in his Feynman Lectures said that the universe is like a game, whose play we observe and based on our observation, we try to deduce the game's rules. This is the best explanation of the idea conveyed here. We did not make the game, we observe and deduce.