The issue is that most native date input pickers use a calendar, which is not always useful for some situations. If I'm planning a vacation, picking start and end dates, sure that could be handy. But if I'm entering something like my birthday, how does the calendar help me? It doesn't really help me to know that I was born on a Tuesday. There is one specific correct answer that I know by heart, I'd rather input it directly than page back in time a few decades and find the right square to click on.