Not just that, simple laziness as well. People get in, do a couple of tours, figure they've "done their bit" and relax, forgetting that they made a commitment to serve.
Additionally, you have too many people for whom their version of serving is sitting at a desk doing office work 9-5,they feel like they don't need to be fit. Their malaise rubs off on others.
If they're not meeting the standards I'm all for them being booted out.
However I wish the same was true in other organisations. The various police forces in the UK recently publicised reports saying that a huge proportion (40+% iirc) of their officers failed to meet the required standards of fitness. They failed to get to 5.5 on the bleep test. At worse, though, they were placed on disciplined. Not one has to face leaving.