Once again, generalizing Shichibukai does not work, to some of them it definitely fits (Croc, Moria), to some it does not.
1) Croc and Moria were beaten, they learned that for them there is an unreachable wall (Yonkou) and got broken as consequence, being only shells of their former self. Went back to Paradise, a sea in which they belong to the strongest ones regardless, and sought alternative ways to power, Moria with relying on zombies, Croc on Ancient Weapon.
2) Doflamingo had the same goal (it´s no coincidence DD, Croc and Moria were shown during Roger´s execution, they adopted the Pirate King goal, albeit with a different interpretation than Roger), also understood he can´t overcome the Yonkou on an individual basis without even fighting them, hence tried sneakier ways to do it.
3) Kuma is Shichibukai by force, he was not a pirate in the first place but a Revo and king, until he was made an example of for disobedience.
4) Hancock has no interest in the New World and Pirate King, she became a Shichibukai to protect Amazon Lily since what they were protected by in the past, meaning Calm Belt, was not working anymore due to Marines´ new technology.
5) Jinbe became a Shichibukai to improve the relationship between fishmen and humans/World Government, to fulfill Otohime´s and Fisher Tiger´s wishes, he had no interest in PK and New World until Luffy came along.
6) Mihawk´s reason for joining is still unknown. Anybody´s guess, mine would be to chill without constantly getting disturbed, similarly to Hancock. He did not lose against Shanks, and has not met Whitebeard, no idea about BM and Kaidou.
Buggy is clear. Blackbeard became one to fulfill his plan. Weevil due to his mother who is money hungry. Law followed his plan.
and the rest of them CAP also
