They cannot leave until their contract is up. They literally cannot leave as soon as they've learned if they still have years in their contract. Additionally, the military dangles a lot of incentives to re-enlist, and people who have spent a lot of time in the military sometimes have trouble adjusting to life outside it. But yes, people will leave. There's always more. You would be surprised how many bright people choose to join the military even though it doesn't make economic sense for them.
I'm on mobile and copy paste isn't working, but enlisted pay grades are publicly available. They are much lower than SV salaries and they are set in stone.