Great to be rich

Well, rocks are quite heavy

Because a celebrity with a busy schedule can walk into a public gym?

Plus there’s equipment preference. Even if you go to a chain gym, some locations have better equipment, some might not have the equipment you need for your routine at all, and then there’s the matter of waiting to get to use equipment wiping down, hunting weights, etc. there are also privacy concerns. Many people like to focus on a work out and not be bothered, but the rock is pretty good natured so he’s between a rock and a hard place of telling people to buzz off or shaking hands and giving advice, listening to bro stories and such- then the fact that he might not want his workout plan to be public or his weights or reps. It may sound funny but people judge on those things, people get self conscious, and people sometimes take it as a serious secret. Also- if you’re on AAS you don’t want to gear up around strangers, and your work out can help determine if you’re natural or not. So there’s that too.