It's probably easier to lip-sync to music, especially a song you've heard (or "sung") 100s of times. Delivering a line that you've only said a few times before, with exactly the same cadence (without a backing rhythm) and intensity is hard.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris