alt Hacker NewsThe answer is UX designers and marketing people. I will yet have to work in a web project where any UX designer will be happy with what the native browser has to offer. Notable examples are date/time pickers. Look at any of the Top100 websites and you will find custom date/time picker designs. And I would also say, with standard browser built-in dialogs you could not rebuild/model those pickers to match what the Top100 websites use.
Not sure why you're being downvoted, this is basically the sole reason the web won as a development platform. You aren't tied to the platform's idea of what components should look like or how they behave, by default. You can take the Figma screens and make them exactly.