...but i do think it's funny how all the games i play and mod run on their own homegrown engines.
minecraft did it out of necessity, i guess; celeste's is iirc an improved version of what towerfall used; and all zachtonics games share the same internals, though the actual content is different enough to make code reuse beyond "a scene" or "a texture" hard (e.g. "timerate" isn't a consistent concept, let alone "placeable object"). and god knows what terraria does, haven't spent long enough on it to know.
the most pleasant as a game engine has definitely been celeste's, but while it's supposed to be source available, i don't know whether the download is still up or if it's even the newer version used for the game.