linguistics question about english plurals

So, why 'deer' and 'fish'?

I've looked them up etymologically, and they're both credited as coming down from proto-germanic. But I looked up proto-germanic and there is a plural form that is different-sounding than the singular. Looking them up individually I saw that there is "deers" as a plural, but it's considered archaic.

I know that it's possible and acceptable to say "deers" and "fishes", but I'm just curious why the more normal plural form sounds the same as the singular form. When I tried googling the reason the only answers I saw were "Because that's the nature of the language, duh." Which is a boring-ass explanation. What do you think, /lang/?

You must tell me first why the plural is also "djur" in Swedish.