Try to put yourself in Casey's position for a moment.
Let's say she's killed her child and disposed of her body. How does she get away with it?
First, who does she have to worry about? Not any of her friends - they're all temporary anyway, and she can always tell them that Caylee is with the nanny, or her parents. They didn't seem to think there was anything amiss about that story, so she was OK there.
She had to have known that Amy would be livid about Casey stealing her checks even if she'd gotten away with stealing her cash, so Amy was used for what she could supply ($) and was otherwise disposable. Tony was fun, but he would have tied her to Orlando so his use to her was as a crash pad to keep her from having to go home. Disposable. Ricardo filled in with a crash pad when Tony inconveniently left town for a few days, and Jesse was used for his shower facilities. Disposable.
The biggest stumbling block was her family. No way would they believe her BS about a suddenly missing Caylee, and she couldn't dispose of them (although I wonder about that, what with the signing over of the mortgage story she was feeding her friends..).
Casey had to leave town, and make her parents believe that she had Caylee with her until she had enough distance between her and them to disappear.
Mark Hawkins was probably the only person she knows that is so far away from her parents' reach that it would buy her enough time to think of yet another story to explain no Caylee.