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One thing might be: In a paid job you can focus on your job. As founder/freelancer you have to handle everything. Some people are good in diving deep but slow to context switching.

In best case they can pair with a co-founder that has a good intuition about „good enough“ and handles all the chores. But then a co-founder also adds its own set of problems.

Most people are more motivated by external factors than internal ones, so they simply work better when they are accountable to someone else. When you are working on your own, there isn't usually anyone making sure that you are actually getting things done.

It’s easier to risk failure at things that aren’t important to you is one example. A great book explaining all the reasons why is War of Art by Steven Pressfield.