why wouldn't someone just hire a developer to do this?
Because developers are relatively expensive.
Because hiring anyone requires identifying someone who can do the job and trusting them to do it well. And even before that, you have to figure out what the job actually is.
Because hiring anyone requires some sort of contract, and that has legal implications that might require approvals etc.
As a developer with the required knowledge and skills, these services offer little value to me, but I'm not their target customer. That knowledge and those skills are the result of decades of study and practice, which is experience that most people don't have, and for all of those people the cost/benefit comparisons are going to look completely different.
Because developers are relatively expensive.
Because hiring anyone requires identifying someone who can do the job and trusting them to do it well. And even before that, you have to figure out what the job actually is.
Because hiring anyone requires some sort of contract, and that has legal implications that might require approvals etc.
As a developer with the required knowledge and skills, these services offer little value to me, but I'm not their target customer. That knowledge and those skills are the result of decades of study and practice, which is experience that most people don't have, and for all of those people the cost/benefit comparisons are going to look completely different.