Click to view I've always imagined that freed from the ridiculous rules of Bowser's ridiculous rule, Mario's enemies would take the plumber down in seconds flat. If only Bowser had paid more attention to those Super Mario 2 minions it wouldn't have come to a head quite as quickly, but you can't hold back inevitability. It's inevitable.
@zenpoet: Because ghosts aren't supposed to be visible things. They hide so as not to be seen. If Mario is looking away, that's their only chance to haunt him!
It's actually really easy to retroactively explain all of the reasons why enemies didn't just go straight at Mario (most reasons being "they are just dumb animals") but then you'd have to ask why Mario, Luigi, and the Princess were the only humans in the Mushroom Kingdom and that's where everything breaks down and you have to admit "it's just a game."