Why are smith machines crap?

They arent,, they best for auxillery workouts instead of core workouts

I don't know why smith machines are crappy in particular, but I call bull on most workout machines. They are useful for specialist bodybuilders who want to target a specific muscle and pump it to llamaland, but people who workout for strength and fitness, going heavy on machines is often a bad choice for the same reason. You cannot expect to get a workout that cover a lot of your smaller muscles if you can only follow a pre-determined path for lifting your weights (or whatever you are doing). It is, in my opinion, much better to learn how to actually lift a weight and use freeweights instead.