There’s pretty good in-universe explanations that are probably more in line with what you wanted out of an answer but also, it’s clear that it is because it is her role both literally within the FBI, but also for writing purposes, to be a foil to Fox Mulder.

It actually worked really well in the early episodes. A classic duality, characters of opposing extremes brought together. Dana continually see things that challenge her rationality and she has to grapple with that while maintaining what she sees as a duty to remain grounded and offer the possibility of the explainable amidst the seemingly inexplicable. The apparent erosion of this level-headed front in the face of the extraordinary and supernatural week after week was initially a point of interest and development in the show.

The problem is that this established the dynamic between Fox and Dana and it was that, that made the show great so they had to keep it up but as there seemed to be no over-arching multi season arc planned they had to keep this going long after it still made any sense. This is especially evident when you see that attempts were made to carry on whole season long arcs while at the same time keeping the Monster of the Week episodes in between story episodes, so Dana would, in one week acknowledge her own direct personal experience and go all-in on taking down the conspiracy and seeing them aliens, and in another week somehow be totally skeptical of Fox’s latest crazy supernatural crime solving theory as if it were the first season all over again.