Comment by WJW

Starting an agency is the easiest part. Then after that comes the never ending grind to find and retain customers, who will never pay on time, refuse to budget time for frivolities like "testing" and "refactoring" and who will mercilessly shop out the work to as many of your competitors as they can find. Developer agencies have existed for decades and the reason that not everybody is rushing to work for them is because they are generally very poor place to work for compared to regular companies, even as a partner. They're often derogatorily called "body shops" for a reason.