Implementing a full featured web page renderer is a massive task. Correctly handling HTML and CSS and JavaScript and Canvas and animations and etc, etc, etc...

Implementing a full featured web page renderer is a massive task. Correctly handling HTML and CSS and JavaScript and Canvas and animations and etc, etc, etc... Then test is against the top 1,000 websites and ensure that yours renders the same as the popular existing browsers, otherwise people will complain and stop using yours because it is different. Then maintain it as new standards constantly evolve over time. Finally your will have a codebase that you cannot sell because browsers are free! Even Microsoft have given up and Microsoft Edge is going to switch from their own renderer to using the open source one from Chrome.