Because science has failed us.
Okay, that's not quite what's going on. Science has actually provided us with an explanation as to why humans can't do cool things like stick to walls. Or whatever else a Spider-pig does.
It's because we're too big.
According to biologists from the University of Cambridge’s department of zoology humans are too big for Spider-Man's stunts. Unlike spiders, which are pretty tiny compared to us, we would need a LOT of adhesive to stick to the walls (via).
Is that science bugging you?
Since ants are so tiny, only a small percentage of their body needs to be adhesive. But humans need about 40% of their bodies to be adhesive in order to actually stick. Which means you'd need some very large hands.
They did mention that it would be possible for us to climb walls- just not like Spider-man. We'd need some special technology to give us the sticky surface area we need. So... Doctor Octopus. Sounds good to me!