Browser-based EPUB and FB2 rendereres are pretty straightforward to build. You don't need a free implementation to embed it in a browser, you can always implement it from scratch if you are interested. You don't even need a team or too much time, a single developer is enough. I don't know much about MOBI/AZW3 but I doubt they are much harder. PDF is already there. Another useful format is DJVU and there have been well-known browser plug-ins for it.

I would love to see all these (not just PDF) to come with the common browsers by default. It seems sad to me they don't. The only browser that actually took care was the old Edge. I was very disappointed they decided not to port this feature to the new Edge.