If Hair Loss Is Not From Environmental Factors, Why Does Hair Only Fall Out On Top?

My question is that when men are bald why is it usually only on the top of the head? it seems as though environmental factors should be considered as a cause. Sun, wearing hats, showering etc.

Now i understand that most causes seem to be genetic but it just makes me very curious why it would not be the whole head.

The sun, wearing hats, showering, etc. will not impact hair loss. It is all in the genetics. If I knew the answer to why genetic hair loss only impacted the top of the head (not counting diseases like alopecia universalis), I could be a rich man. Unfortunately, the answer is not known.