How many major browsers do you know? How hard it is to implement your own? How hard it is to just read and roughly audit the source code? How much control do you have over the features of the browser?
Well, several are open source. I know that they have vast and complex code bases and are extremely hard to understand or modify but it is possible and diverse communities are working on them.
For me the centralisation is at the search engine and the content generation; both have narrowed and narrowed and narrowed.