Short answer: because they are BORING!!
Please let me preface this blog by saying that I'm a natural bulker! When I was 8 years old my doctor nicknamed me "the ox" because my back was so ripped! I certainly have never had a problem adding muscle! I come from a place of big trying to get little.
Last week I was with a client and she was remarking on how we really don't use machines all that often. I thought it would be a great blog post to explain why.
It is my belief that one of the main reasons why machines were even invented in the first place is because they prevent liabilities for gyms. A machine naturally puts people in a position where their range of motion is limited, making it much less likely that an injury will occur. At most gyms trainers really don't have that much training - a certification can be obtained in a weekend! It is way faster to train someone on a machine than it is to cover proper form with dumbbells! Machines are essentially idiot proof!
I stopped using machines in my own training after taking a class in University called "Physiology of Exercise Training Prescriptions". One day our prof drew a pictures of a leg press on the board and then went on to talk about how strength is totally about angle of the muscle trained. This was a "aha! moment!" for me. I'd never thought of a muscle having multiple angles of strength in it before. Basically, if you're training to strengthen your legs for a specific sport if doesn't do you any good unless you're constantly mimicking the exact same angle your sport requires. For example, in rowing a leg press would be more beneficial than a squat because you're at a similar angle when you're rowing.
When you train with machines you're training the same angle over and over again. Your muscles are constantly ripping and repairing in the exact same place. If you picture muscle repair like adding plasticine to the rip then that mound of plasticine is going to get pretty big! Which is great if that's what you're after but it does very little to improve your functional strength, and does nothing to lean out your muscle mass.
I like using dynamic bodyweight exercises, dumbbells and mixing that training in with high intensity cardio to avoid adding lots of bulk and keep lean!