Is there any reason with Spiderman's strength he can't just kick someone so hard their neck snaps or their skull explodes or their head flies off their body like a soccer ball? If so why does he still have trouble with normal human villains like the Kingpin? Can't he just literally tear them apart if he wanted to?

Well, that sure sounds like Spider-Man, doesn’t it?

This is mainly a function of what i think of as “powers drift”, and the handbook is as much to blame as anything else. Back in the day, Spidey was depicted as being strong, but not crazy strong—a trio of normal guys like the Enforcers were able to give him a good fight. However, when it came time to quantify Spidey’s powers for the Handbook, the researchers worked out the maximum amount of weight that Spidey had ever lifted—that huge Doc Ock machine off his back in the Master Planner story. And that’s what they put down as his maximum strength rating. Fine so far as it goes, but that story makes it clear that what was involved was a singular and superhuman effort of will, that heroic moment when an athlete pushes past his limits. But, because the Handbook said that Spidey could lift ten tons, that became his default “normal” strength.