I don't think this is true. I think what you may be thinking of is many hit games did not create their own game engine.
Counter-Strike
Every MOBA that exists (DotA, LoL, HoN, etc)
Team Fortress
Killing Floor
PUBG
Natural Selection
Undoubtedly, many more that I can't recall off the top of my head.
You don't need to make your own engine to make a hot game from a mod though?
No many hit games started as mods. League of Legends is the one that immediately jumps to mind, but I know there are many more coming from Quake and Doom mods etc.