Because English is a terrible, thrown-together, inconsistent language, and nobody even using English words can keep completely straight what words are spelled "ie" or "ei".
There's a rhyme they teach children who are learning to read, "I before E except after C".
There's a rhyme they teach children who are learning to read, "I before E except after C".
https://en.wikipedia.org/wiki/I_before_E_except_after_C
No matter what extra verses they add, there's still plenty of exceptions. English spelling is not consistent no matter how many rules you add to it.