That is a requirement for a single piloted military high performance aircraft. The reason is because you don't want the pilot trying to change frequencies during such a critical phase of flight, particularly student pilots. If you have traffic, you should have resolved that before clearing them for takeoff.I Previously working at a fighter base, and it was always, "Rwy XX, wind 230@05, cleared for takeoff, change to departure.
Why would you say, "Change to departure" first? Before handing someone off you wouldn't say, "Contact Columbia Approach 126.5" followed by "Traffic twelve o'clock 5 miles....." As soon as you say change to dep, that pilot is going to push the freq toggle switch and assume he's cleared for takeoff.