If you can't complete the prescribed exercise with proper technique, you will fail. So that's two parts, getting it done and doing it right.
You can learn all the techniques in the world, but if you don't have the musculature to do the task properly you're screwed. Lift/agility testing is designed to weed out the people that will put themselves in jeopardy and has the not to be overlooked benefit of eliminating the people who will make their partners do more than their fair share.
Height also plays in here, but sadly it's not routinely tested for. Something about discrimination. I hate working with super short people. Even if they're strong, they are still at a significant disadvantage when carrying a patient, a disadvantage I have to make up to get the job done.