1) McCaffrey's teammates (and current players now) understood his position going into that one final game, know that he has given everything to the program and has been a great teammate, and that risking injury for a bowl game is not worth it for their future careers.
2) The bowl games are pretty meaningless, especially if a guy has been to a number of them early in his career. The bowl games were created to bring teams from different regions of the country, as a reward for a very successful season. Now, even average teams makde it to a bowl. The same guys on ESPN work for the network who created and own many of these bowls. The expansion in number of bowls in the past 10-20 years was done to create more content for ESPN over the holidays. ESPN also has a huge stake in the playoff, and do everything in their power to generate interest in the playoff. ESPN has created the thought that if a team doesn't make the playoff, that everything else is meaningless.