Couple of thoughts.
First, the size/strength differential between 17-18 year old college football players and senior college football players is pretty wide. There are some college football players who absolutely should not be put on the field as freshmen. When you get to the pro level, I think that gap narrows. Even undrafted NFL players are getting into games (mostly pre-season) and are not absolute liabilities. ****, UDFA Pyburn just got 2 sacks. And also when you get to the NFL, the size/strength differentials between rookies and 10-year vets is also narrower, so maybe fewer injuries?
Second, I think college football (some of it due to the old Initial Counter and Counter scholarship rules) tend to carry non-contributing players for longer. I don't think anyone ever said "hey, we might NEED that 85th player on Saturday", but I coaches tended to carry guys on the roster thinking that they might contribute next year or the year after.
Third, once you get to the pro levels, you have "culled the herd" so to speak, and you have the best of the best. Also, you have an even higher level of strength/conditioning/nutrition and a team of doctors to deal with every form of injury. So whatever is happening in the NFL is keeping guys closer to the levels of readiness and health that do not necessitate a larger roster. You and I are old enough to remember the old days, when NFL rosters were even larger, and somewhere along the line the cost-benefit analysis of keeping (and PAYING) more guys to "be ready" just didn't make as much sense.