In the standard model, the origin of the values of the coupling
strengths is mysterious. It takes only a few seconds to understand that GUTs do not solve this problem; GUTs just shift the problem away by one level. The same is true for supersymmetric GUTs.
Also the number of particle generations is not explained. Of course, one can claim that the number comes from the GUT Lie group [insert your favorite one here]. But what determines the Lie group? GUTs do not provide an answer. Again, the solution of a problem is just shifted away, into a region of higher abstraction.
GUTs also have another tiny issue: all GUTs that have been tested contradict experiment.
Short summary: GUTs do not solve any problem of the standard model and they do not agree with experiment. So why are they popular? Because there is a lack of better ideas. But if an idea does not work, we should drop it, not continue to pay attention to it.
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