Most marketing people don't. Basically, I have a lot of experience growing and managing high spend ad accounts at this point. Once you have that experience and skill, you're in much higher demand. I have a lot of references saying I'm good, so I can negotiate hourly + % of ad spend. Pareto's law is in full effect in online marketing, so 10 hours a week with a client can get them very similar results to if I worked for them full time.

For example, one of my clients I grew from $0 Facebook spend to 6 figures Facebook spend. They're happy because it's profitable (printing money for them), I'm happy since I get 15% of ad spend. It doesn't take that many clients like that before you're making $30k+ a month.