As a former Recon Marine I can attest that there are two reasons that people would drop out or be dropped for selection.
The first wave of drops is how you describe. It is people that don't have the basic physical performance to continue. This in my perception accounts for a relatively small number of drops due to everyone going through prior screening.
The second wave of drops is very similar to the first, but the root is different. The failures are typically for physical reasons, but the source was mental. A lay thing to say is "He didn't have the heart." These people are typically labeled as "badge hunters" as they are looking for the bravado of the title, but honestly don't enjoy the job itself. The wrong types of motivation will quickly weed themselves out.
The first wave of drops is how you describe. It is people that don't have the basic physical performance to continue. This in my perception accounts for a relatively small number of drops due to everyone going through prior screening.
The second wave of drops is very similar to the first, but the root is different. The failures are typically for physical reasons, but the source was mental. A lay thing to say is "He didn't have the heart." These people are typically labeled as "badge hunters" as they are looking for the bravado of the title, but honestly don't enjoy the job itself. The wrong types of motivation will quickly weed themselves out.