An OT has 4 years graduate school plus internships and externships. There are significant early barriers.
I think that in addition to that, it is not possible to practice without passing the exams, similar to doctors.
You can work as an accountant with passing the CPA exams and many do so successfully. You can work as an actuary (see DaMoshe) without passing the exams.
You are focusing on the wrong data in determining why certain fields have higher pass rates than others.