Modern UIs with support for themes, complex interactions, multiple screen and pixel formats, and every language spoken by Homo Sapiens since Gobekli Tepi was built are large and complex. It's not avoidable. That is the problem domain. If you're doing less than that you'll regret it when more and more users start requesting features you don't have or complaining that your product doesn't look right on X or with X language/font/etc. That's my problem with all these ultralight immediate mode UI libs like nuklear. It's like going back to MS-DOS or CP/M and saying "wow that's simple and fast!" Yeah but it lacks a lot of stuff you will need.

The web's rendering layer isn't perfect but it's better than many alternatives and isn't going anywhere, so putting a lot of effort into making it more efficient and robust is very logical.