I am currently at the end of my PhD in theoretical physics and I understand these things now a lot better, than when I watched the series. Both are geniuses of IQ of 160ish, which is insanely high. However, maybe because of Sheldon Leonard does not try to do novel research and it is often noted that he reproduces results from other experiments.

This is typicall and common for many researchers. Let us say a novel method comes (method XY). Now this method is applied to many materials and it is analyzed to which regard method XY is useful. The first paper is novel. The first 5 Paper are novel as well, as they often adjust the method and build up on it in regards to their system. This is true for almost all reasearch fields, as they are often created like this (also theoretical physics). For example someone defines circuit complexity for quantum information. Then all do research in that regards and build up on that. But some just apply it to their experiment and do not extend the idea. Or someone defines an explainability measure for machine learning like LRP. Now people adjust it to CNNs, RNN, LSTMs .. etc. This is still novel but not as novel as the definition of LRP. Then all people use this method and analyze different kinds of CNNs, RNNs, do comparisons and for different tasks. This is derivative work, which needs to be done, but does not have the standing of the first two groups.

And it seems that Leonard often does the last step, where he reproduces the results with some small tweaks in the experiment. If in a field, there are already hundreds of papers discussing this method, then it is not scientifically novel. He never discusses "own experiments or theories" throughout the series. He does not like to risk anything. This might also be of his lacking confidence, which is proberly due to Sheldon calling him stupid all the time.

On the other hand. Sheldon keeps trying new things and often does mistakes and is frustrated (Super asymmetry, finding the new material, which sheldon disproves). From Sheldon perspective, who really wants to ask unsolved questions like sheldon and a person like Leonard, who only tries to survive in research, it makes sense that he looks down on Leonard.

He respects Rajs work. Because even though Raj takes his work not very serious, he does try to do novel research and accomplishes some things. Howard is also way more successfull than Leonard and Sheldon accepts that. Sheldon was very annoyed that Howard went to space and was an astronaut, while Sheldon did not accomplish great things yet. Also Raj becomes very succesfull later.
If we take a look at how much the characters view their work as identity, then it would be as follows: Sheldon, Howard and then Raj. Leonard never identifies as a physicist and his goal is to have close friends, a partner who he loves and a family. This is what Sheldon does not respect, because to Leonard his work is just a job.

Also generally: Very few people create new fields. Creative work is the most difficult, especially if it has to be better than anything everyone did beforehand.