When will people realize you can't just lift up a manhole cover?

One of my favorite quotes of all time comes from famed director Alfred Hitchcock, who noted that you can make an audience believe the impossible, but not the implausible.  I try to keep this in mind while writing, and it's a tool that I wish more writers - in comics and other mediums - would follow.

In comics, for instance, I have no trouble believing that Superman, as a character, can exist.  Alien from a doomed planet, just happens to look human, gets powers from the Sun being a different color than his...I'm fine with that.  Bruce Banner gets bombarded with Gamma radiation, and instead of, I don't know, dying of cancer, he turns into the Hulk.  Beyond where he gets all that extra mass he's lugging around (and how his pants stay on), I'm fine with it.  Absolutely!  That's integral to the plot, and I can suspend my disbelief for things like that with no problem.

However, I have to draw the line when it comes to events that are just too implausible, even in a world where the Flash can run near the speed of light and not catch on fire from the friction or suffocate from the lack of oxygen (I know, I know...he's got his "protective aura".  Whatever that is.)  I absolutely cannot stand when a character, perhaps one fleeing some imminent danger, decides to take to the sewers as a means of refuge or escape.  This character spots a nearby manhole and makes a break for it.  He or she slyly LIFTS THE MANHOLE COVER out of place, takes the ladder down into the hole, then slides the cover back into place, slipping out of sight.

Don't people realize that manhole covers weigh, like, 300 pounds (136 kg to our international readers)?  Seriously, go outside now, locate the nearest manhole, and try to move it.  Give it a try!  Unless you have something that gives you a decent amount of leverage, it's not going to budge.  If manhole covers were really as easy to move as you'd see in a movie or in a comic book, people would steal them all the time.  College students would take them and hang them up on their walls like they do with street signs.  People would have fun and toss them around like an oversized Frisbee.  Ever wonder why this doesn't happen?  Because it's near impossible to move them.  When work crews come and need to remove a manhole cover, they have tools to facilitate this.  They don't just bend down and grab it.

The above pictures are all from comics I purchased in the last month (though one is, admittedly, a reprint).  None of the characters so nonchalantly grabbing the manhole covers in question is imbued with super strength.  I suppose you could make the case for the Ninja Turtles being stronger than people, but even that's a stretch.  My point is, these writers are all talented.  They work on popular books and characters.  Why, then, do they write such ridiculous, implausible scenarios into their books?

I want to write a scene in a book one day where a character tries to go into the sewer by lifting a manhole cover, and instead of gaining instant access, he gets a hernia from the effort.  Also, I want to write a character who shouts "people only use ten percent of their brains!", and immediately have that character get hit by a bus.  That's one of the most unbelievable phrases I continue to hear and see in various forms of media.