My own reason for not doing it myself lately... the built in browser behavior is, or should be the expected behavior for what the user is using... I'm not going to "fix" that for them. As much as I don't like the various inconsistencies between browsers on Date input controls let alone time.

It's something I don't think anyone gets right.. I think MS Ajax Toolkit came closest imho, but that didn't include a prescriptive time input and too small for tablet/phone input.