As the ProTac demonstrates, fitting a mech switch into the head creates something very ungainly. Putting it at the tail end would lead to terrible ergonomics for almost all uses except when actually in a headband.
My $0.02, if you’re concerned about standby drain in the relatively short term (a few months or less), pony up the cash for a Zebralight or other light known and tested to have low standby drain. For long term, simply unscrew the tailcap a bit.