Why does the UFC has so few injury replacements?

I've noticed that compared to other promotions, many less UFC fights have fighter substitutions or are scrapped due to fighters getting injured. This is despite UFC fights being booked much further in advance than other promotions.

What's the explanation here? IMO, the most likely reason is that UFC fighters fight through serious injuries because of the relatively high pay and exposure they receive. Is the UFC a living, breathing rape tunnel?

Guys definitly do fight in the UFC with injuries that would take them off of a smaller level show for smaller pay. That is definitly one reason.

Also, highter level fighters train smarter so they get injured less in training.

I feel like it gives the top ufc fighters an advantage because they can back out of a fight or not take a fight knowing they will get many more chances while an up and coming fighter needs to make the most of his opportunities.

 Agree with rape tunnel theory.

with this guy waiting for you with lube at the end of it.

Rape = no lube IMO