because if you already have to write javascript for the datepicker, why figure out if the browser supports html5? why not use the datapicker you just wrote (using yui or jqueryui or whatever)?
to do otherwise just adds more work, not only for detecting the browser, but for making sure the page layout and so on works in both versions.
Because everyone who does do this will provide a better user interface than you do. You can choose to make your life easier or your users experience nicer.
Personally, my strategy is simply to not write HTML or CSS. I write to a DSL that is implemented based on what browser is being served, so there are no sniffer scripts in the pages themselves. True, this does mean the same, otherwise static pages, may need to get generated over and over, but this is where caching comes in.
to do otherwise just adds more work, not only for detecting the browser, but for making sure the page layout and so on works in both versions.