because that's f*ck*ng French ! ahah JK
As you might know, French is a compound of latin, greek, it then evolved for centuries, several regions got influenced by neighbooring countries or immigrants.
There are quite a few words that are spelled in a way that does not seem to match the way they're pronounced. That's one of the characteristics of our language.
Good news is that, most of the time, these tricky words are the most used ones. Like "femme" is quite a common word, isnt it ? Some numbers, basic verbs or adjectives are tricky ... how did you feel learning être & avoir ;) ? But yeah, you meet these words so often, as they're common ones, that you'll just remember them pretty soon ;)
Good luck !