Why can't everybody use the same software?

Why do we need so much programmers when everybody can use the same source code ?(few coders to modify it a bit here and there). Why we need million different website when we can all shop at amazon?(ok we need few more, but no with lots of zeros in numbers). I know we need variety, but common? Isn't simpler and... show more Why do we need so much programmers when everybody can use the same source code ?(few coders to modify it a bit here and there).
Why we need million different website when we can all shop at amazon?(ok we need few more, but no with lots of zeros in numbers).
I know we need variety, but common? Isn't simpler and streamlined better?
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