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.
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.