Is this just a function of how long it takes to recreate ~80% of the functionality?
It doesn't matter if it takes half a decade to implement the last 20% of the feature set and iron out all of the bugs and corner cases, if you can bang out a prototype in a long weekend, someone is going to do that over and over. Meanwhile, if it takes nearly a year just to get something half-working, the existing open source codebase has to be pretty bad for you to not at least fork it.
It doesn't matter if it takes half a decade to implement the last 20% of the feature set and iron out all of the bugs and corner cases, if you can bang out a prototype in a long weekend, someone is going to do that over and over. Meanwhile, if it takes nearly a year just to get something half-working, the existing open source codebase has to be pretty bad for you to not at least fork it.