Just off the top of my head, Celeste, Super Meat Boy, Fez, Spelunky, Dead Cells, Braid, and The Witness all use custom engines and are widely considered some of the best indie games of all time. Moreover, I think much of their charm and uniqueness stem directly from the amount of custom engine work involved. Writing your own engine lets you focus on the details that make your game stand out in a crowded field.

Your assertion seems silly.