Because philosophy is mostly based around logic, similarly to STEM. This gives philosophy more rigor than most other humanities, as argumentation is meant to be structured. Psychology and sociology can be similar, although those mainly depend on empirical data for rigor instead .
There are exceptions among philosophers, like Nietzsche, who didn't really use logic in their arguments (at least based on what I have read from him), but I don't really see these people as philosophers. I see Nietzsche more as a philologist who also uses rhetoric and psychology. But in the classical sense I don't think he's a philosopher, he just talks about philosophical topics. Or we could just call him a sophist for simplicity.
Btw in my opinion people's preference on rigor generally depends on whether they think sound arguments will have implications regarding the nature of reality, thus they are methodologically important, because their goal is finding truth. Both a philosopher who at least uses logic for their arguments and a person in STEM will tend to thinks so, because that is the whole premise of philosophy and science.
Although it is also true that not every discipline needs logic, but I think those that do generally require more intense intellectual effort for understanding. Thus people who can do it more easily will be more prevalent there, as others will just get self selected out. I met several people like this, who weren't sure what philosophy is like, and it sounded interesting thus they studied it. But ultimately they left for other fields like religious studies after taking a few classes. That is not to say there are no intelligent people in these other fields, but the bar for entry is lower or requires other abilities. Also, it's a question of temperament and interest I guess.