Because free weights in general are much better than any machines if you have the option. Machines will teach you bad form and won't help you build up any stabilizer/supporting muscles muscles. They will also make you think you are stronger than you really are because you can "lift" more weight. In reality you can put more weight up because you can use other forces and muscles to help, plus it's on a track and only allows motion in one direction.
For instance, if you are doing bench press on a smith machine, you can push forward and up to help you get it up if you start struggling. If you did that free weights, the bar would end up on your stomach.
I jacked up my back like 10+ years ago doing squats wrong because of bad form and habits learned from using the smith machine. I also used to do shrugs on the smith machine if all other racks were taken but now i never have that problem because I'll use flat bench or decline bench if all racks are taken. That gives me like 8 possible options to do shrugs with at my gym so there's always something open.