The 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.