I have never been a true retail trader, but I had a stretch of downtime (was in a hospital for 6 months and had to deal with boredom) when I traded for myself. In my opinion, there are a lot of considerations that come into play.
(a) As a fund PM it's much easier to get paid a lot of money because of access to capital. For example, if a PM is getting 12% of PnL net of costs and can make 10% return on capital, he would need to manage a measly 100mm to get paid a ~$1mm. As a private trader, he'd need to have at least 10 million in his account. Unless a PM had the right parents or won a lottery, it's a LNW number that can only be hit at the tail end of the career.
(b) There are a lot of types of trading that require institutional infrastructure that is pretty hard to get as a retail trader. This said, it's certainly has been getting easier over the recent years and there is a fair number of "independents" who run strategies that are very similar to that of institutions.
(c) Skillset is somewhat different and returns would be much worse for an institutional PM. It is infinitely easier to make good return on capital as a retail trader than an institutional guy. E.g. there are plenty of specialized capacity-constrained trades that would produce 25-35% per year on $1mm, but usually impossible to scale that up. If you a managing a few hundred, you gonna end up thinking about impact, about crowding etc, stuff that a retail trader would rarely consider.
(d) Its undoubted that a median institutional trading is a better at generating PnL than a median retail trader. It simply comes from better training and knowledge base. However, a retail trader is not burdened by compliance, restricted mandate or onerous risk checks - she's also not required to run stuff at a certain capacity all the time.
(e) Finally, you have to realize that most funds would have a prop book as well. That means if a fund manager or a PM has a strategy/trade idea that's too good to share, he'd put it in the prop book. Not very ethical, but certainly not illegal.