Because some dude or gal somewhere some time ago thought it would be funny that generations to come would need to learn irregular forms of some random words! Why would grammar rules apply all the same to all the words, not that logical is it? /s
You know nothing, Jonh Snow (seriously though, from a russian point of view English has very few of them)
P.S. Russian is much easier to spell than English, but its verbs of motion are a real clusterfuck.
P.P.S. I have met more than one Russian who said, "Only foreigners decline numbers correctly."