One of the hard lessons you learn after decades of coding is how large the burdens are of maintaining software compared to the initial work involved. It is my impression that this has gotten much much more true in the modern age of the internet. And for some programs, like browsers, this is even more true.
And for some programs, like web browsers, there is a huge amount of work involved in maintaining the ecosystem of addons, etc., and there is a real competition for the attention of peripheral players in the ecosystem (addon coders, etc.), and people would need to see some real long-term commitment to a new project before they invest their time migrating to something new.
So what we have in the world of web browsers is a situation where it's technically easy now to build a "new" browser using open source components and libraries (or indeed just forking an existing open source browser), but where people would be foolish to ever adopt your new browser until they were convinced you had a serious long term commitment to it, and had the time, energy, and skill to maintain it for a long period, and the social networking savvy and connections (and monetary backing) to establish and keep a real foothold in the browser marketshare.