While Clark Kent's talents lie in writing and reporting for The Daily Planet. Superman spends his time saving the citizens of Metropolis using his Kryptonian abilities, which includes impenetrable skin, super speed and strength, flight, super breath, heat vision, x-ray vision, and super intelligence. There are also times in the comics where he gains new abilities but they are usually temporary. Despite not using a disguise, most don't often see the connection between the two and here is why.
1. "No Secret Identity" Theory
This is the most popular theory (given in the 1980's) as to why none recognizes Superman's dual identity. Most know that Superman is Kal-El of the planet Krypton, son of Lara and Jor-El. While Clark Kent grew up the son of Martha and Jonathan Kent on a farm in the town of Smallville, Kansas. Due to the public knowing his real name (birth name) and not wearing a mask of any sort, civilians assumed that Superman didn't have a civilian identity. Therefore, if someone did see the similarities between the two, one might simply reason that Clark Kent looks a lot like Superman.
2. "The Glasses" Theory
Despite Clark Kent's glasses being a trope carried from the original Superman comics, writers tried to strengthen the argument that the glasses could shield his alter ego. For instance, in the 70's, the glasses were once said to have been made of Krypton material. This material lets out a small dose of hypnosis that created doubt in Clark Kent being Superman. But this reason was quickly dissolved due to it creating more problems than solutions. However, in 2004's comic Superman Birthright, the glasses were once again used. This time, it was said that the glasses diffused Superman's unique blue eyes into a glass vivid gray color.
3. The "Lex Luthor" Theory
Despite Lex Luthor finding out about Superman's identity multiple times, there are few in which he denied any connection between Clark Kent and Superman despite evidence to the contrary. For instance, once Lex Luthor built a machine that allowed him to find out Superman's secret but opposes because he couldn't believe someone as powerful as Superman would be comfortable pretending to be as weak and feeble as Clark Kent. Much like Lex Luthor, many people who can see the differences most likely think Superman is too mighty to pretend to be clumsy Clark.
4. "They've Been Seen Separately" Theory
While taking the precaution of vibrating his face at super speed and standing farther away when photos are being taken, Superman also made sure to keep his identity secure by having Clark Kent and "Superman" be in the same place. For instance, in the Silver age Comics, robots were used to protect his identity by showing up next to Clark Kent. In addition, both of Superman's Justice League members Batman and Martian Manhunter dressed as either Clark or Superman to help him. Matrix even posed as Clark Kent to provide a reason as to why Clark disappeared when Superman died for a brief time.
5. The Classic "Too Different" Theory
The most popular behind the "no secret identity" theory is the original notion that people don't notice the similarities between the two because they are also so many differences. In both the comics (and the movies, specifically the acting of Christopher Reeves), Clark Kent's posture is slouched, he becomes clumsy and more of a pushover. And while Superman's voice is deep and assertive, Clark is timid and his voice shifts to a higher octave. They behave like separate beings and so for the very few people that know both Clark Kent and Superman, it would be hard to disregard their attitude and behavioral differences simply because they look similar.
