This question relies on faulty assumptions...
- Scientists aren't geniuses.
- Not all geniuses want to be scientists.
Intelligence is simply a measure of how fast you can take in new information accurately and put it to practical use. It has nothing to do with your desires, passion, will, access to information, etc.
Likewise, science is just a methodology that rules out wrong answers, that if followed, whether they be the slowest dumbest, person in the universe or the smartest, most intelligent, they will get to the most correct answer possible eventually, by removing all the wrong ones, if done correctly.
A scientist is someone that does science. This implies that the best intelligence is a type of inherrent science and anyone who is intelligent is on some level a scientist, but that isn't what you mean and there are arguments against this, like we are pretty sure that the brain doesn't use the scientific method to come to the correct answer a lot of times, but rather some sort of multiple short hands that put the feeling that the answer is in a general vicinity and some people just have a happened to be pruned a given way that gives the right answer. That's why the process to get an answer is just as important if not more important as the answer itself.
What you mean is someone who sits in a lab as their job, does research, performs tests, and writes published scientific papers. This is a very limited view on what a scientist is and a lot of the people that fall into this group probably wouldn't fall into the group known as scientists based on what it actually means.
The reason Clark Kent does not have this job is simply. He doesn't want to be a scientist. That's not his passion or what he cares about. He does it, as other people have mentioned, but that's not because that's what he wants to do. I like making games and to do that knowing how to program and build computers is very useful. In making a game I do a lot of the things that a Computer Software Engineer does, or an IT person does. However, that part of what I'm doing is just a necessary evil and not the ends of what I'm doing. Superman, likewise, is doing science, not as an end, but as a means to an end.
Clark Kent is a journalist, depending on which cannon you're looking at, because he has always wanted to be a journalist and so that is what he does. Another reason is that this gives him a human connection and keeps him grounded in the real world which is why in some cannon he does primarily human interest stories rather than big breaking news. And another reason which is oft cited is that journalism puts him in a hub of communication about breaking events which allows him to hear about and react to important things quickly as they come up. He can listen to everyone on Earth, but it would not give him the info needed because he can't distinguish important things from non-important. When focuses his hearing he has to direct it and in doing so he is losing the ability to listen to other places at the same time which makes it an inefficient bottle neck since where he needs to be listening is always changing. With taking a job as a journalist it puts him in a bottleneck where he can get use to the voices for certain things and pick them out much easier and have them funneling and prioritizing information he needs to act quickly. (btw, this is also why the modernized blogging Clark is so stupid, because it breaks the point of him being a journalist if this is why he's there being a journalist)
There are other jobs that do this that may be better or worse, such as police, politics, army, etc, but there is probably good reasons why you would not try to use them as such hubs of information, especially while being a superhero, such as with the army, you can't just up and leave, Politics would put you in a position to be scrutinized a lot, and journalists are less feared than the police and likely get more information (at least they used to be).
If I were to redesign Superman today, he might just rely on Social Media, be an author, and do some job where he interacts with people like a counsilor or something like that. At least if you used the same reasoning as to why he became a journalist, especially since journalism is nothing like what it used to be.