Why do people say homemade nightguards make TMJ symptom worse?

Easy to chew. Most home made "boil and bite" mouth guards are made of a rubber material that although provides great protection againest concussions in contact sports, also causes many people to chew on them as well. A professionally made bruxism splint should be fabricated out of acrylic or some other hard material to prevent additional chewing and clenching.
Not good enough. Most "home" remedies for TMJ will exacerbate the problem. Tmj issues require careful fit, adjustments, and monitoring of the splint(orthotic)to be successful. The "off the shelf" TMJ treatments are usually a boil and bite affair, "the bite must be perfect" (usually missed) and the soft nature of these lead many to chew on them which can make things worse.
People are right! Homemade night guards often make TMJ problems worse. This is because they don't fit properly and are too soft to do any good. In order to work properly, a splint needs to be very rigid and hard, be a custom fit, and stabilize the bite. Only a properly trained dentist can fabricate a well-designed, good fitting splint that will relieve TMJ symptoms.
Not fit well. The nightguard must fit well. sometimes, pharmacy bought nightguards work fine. Try them. You may need a custom mouth guard made by a TMJ specialist.