WF is nothing more than an auto attack that gives passive stacks. Yes he has the hardest hitting auto attack in the game, but that doesn't make him good. Also, block fucks his damage big time because of it. The fact is that other specialists and assassins are just straight up better than he is.
Because a lot of specialists are straight up better assassins than assassins themselves.
Envenom is too good to pass up on pretty much any hero that can get it. I think Zagara, Murky and Brightwing are the only ones that don't get it 100%.