The native ones don't consistently/easily support "mm/dd/yyyy" formats, and many customers want that.
One general design decision about date-pickers is if the target date range is large, such as birth-date, then one needs an easy & obvious way to select or scroll the year also, not just month-by-month.
One general design decision about date-pickers is if the target date range is large, such as birth-date, then one needs an easy & obvious way to select or scroll the year also, not just month-by-month.