The reason is that languages aren’t made, especially they’re not made to be simple or logic. They’re organic and have grown and each language has it’s difficulties and flaws.
German keeps it’s genders because they seem to be natural to the speakers. We learn a language from our parents, develop a right/wrong feeling and understand the rules without having learned them. And so the language continues, there’s no program to it.
English is the exception with no gender to nouns, probably because it’s such a mixed language. Old English still features those, similar to today’s Icelandic. So the English “they” comes actually from three words, in Icelandic those are: þeir, þær, þau.
English might seem simple and natural to you, but believe me, it isn’t. The spelling is a complete mess, Ghoti being theoretically a legitimate spelling for fish Ghoti - Wikipedia. And don’t even get me started with time clauses: going - I went. Uhm, okay… And then: I went. I have gone. I did go. I have been going. I had been going. And that were just the past tenses. You know how many past tenses Swiss German has? One. Also one present tense. And you can use future, but you don’t have to. So two tenses are all that you need. So English can be needlessly complex too, with learners having to remember graphs like this:
