Why is it not recommended to use mouth pieces from walmart for tmj?

Not very accurate: They may make it worse. Think of it as you making your own cast for a broken arm. The accuracy needed to fit properly and the adjustment needed to insure proper bite cannot be achieved unless @ dentist.

Likely wont work: A one size fits all answer will not work for TMJ problems. All teeth and joints are unique to some degree. A custom, reversible occlusal splint is usually recommended for such issues. These can only be done by a dentist/orthodontist knowledgeable in TMJ function and treatment.

Cause more problems: Non-custom mouthpieces may be too thick or not fit properly. This can create muscle spasms, move teeth and change the bite- all leading to a tmj/tmd ailment. Consult a dentist who is knowledgeable on mouthpiece es and consider a custom mouthpiece.

Probably won't work: Probably won't work. But no disadvantage aside from lack of effect. won't hurt to try it. Not impossible that it won't help.