Time and knowledge. Browsers are basically almost an OS nowadays in capabilities. Yes you can build a basic HTML renderer quickly. But anything beyond that just takes a enormous amount of effort and time especially if you want to make it performant and secure. Like it’s very easy to accidentally introduce a vulnerability that can be exploited by someone. Like the last few generations of Nintendo consoles were hacked and jailbroken trough the browser. And that’s a browser build with WebKit by a team of engineers. Good luck doing it on your own, especially without Chromium or WebKit.