Just my two cents:

Most wanted to leave but cannot due to bond. The penalty can quite substantial (im not sure of the amount) if you break the bond, especially so for a fresh graduate who may not have a lot of money to begin with.

Those that stayed past their 4-5 years bonds usually find that they are no longer competitive to jump to other areas or industries (unless you are jumping into sales which can be cut throat). So most unbonded peeps will jump while they are still young.