(On the other hand, the process could be captured by incumbent employees who do have that incentive).
That seems to be way more common than most people want to admit. So many software engineering interviews have very little to do with the job. Worse of course is when there is no process and it seems like the whole thing is just designed to get the hiring manager friends hired.
Companies are not doing it to artificially restrict supply. They have no incentive to raise the cost of labour.
(On the other hand, the process could be captured by incumbent employees who do have that incentive).