TinaT
Here's a link to outline how ligaments 'heal' without surgery: http://www.peraspenberg.com/texts/how-do-tendons-and-ligaments-heal/
Scar tissue replaces the ligament for stabilization. It might not be as flexible and responsive as the original tissue, but it gets the job done.
I've talked to humans with bad knees... the ligament replacement surgeries are complex and not always successful... and that's on a human that KNOWS they need to stay off the knee (i.e. on crutches) to complete the surgical rehab. You mention athletes getting surgery - sure, if their life depends on their athleticism. For "normal" people, R&R (on crutches) is what gets the knee back in working order. And many people shift from jogging to bike rides due to knee issues.