Maybe not "who" but "what is the reason" and I think that answer is
different for different people, but for me, an EAA member who will also
never build an experimental airplane, and who also has only flown in a few
experimental, the answer is TO DREAM.

To dream about what is possible. To dream about new ways to produce
improved results. To dream what hasn't been dreamed about by anyone else.
To see and hear about other's dreams. Non of the great experimental
aircraft would be reality without those initial dreams dreamt by the people
behind them. Also non of the great production and military aircraft would
be reality without the initial dreams behind them.

So maybe the "who" are the dreamers. Dare to dream.

Jim Burns