Because all the points you stated can be played top and for better reasons. If you're thinking wave clear and pushing, it's easier to manage that in the top lane, where the jungler and mid lane threaten to gank you, instead of mid, where bot, top and jungler all threaten to do so.
In the top lane, you'll have an easier time poking and getting in melee range, while in mid, ranged mages will give you a hard time, especially for your barrels.
GP mid has potential, but top lane is still a much better choice for him.
Not really... if he gets one kill early its, over. Especially when he gets his tri-force.
Explanation of Q and W