Good question| I am assuming you mean connective tissue, cartilage, tendon, muscles? In a lot of ways, your question is an engineering/physics question. We are talking "architectural strength and design utility" here. The design is such that the joints need to be cushioned to prevent "wear" and the bones need to be moved in a directed sense - the only way this can happen is for the brain to tell the nerves to tell the muscle (which is connected to the bone) to "move." Another way to think about this is to grab a rubber band and stretch it, then let it go - as long as it's a fairly new rubber band, it will come right back to the size and shape it was before you stretched it. This is what your ligaments and tendons do - they keep tension on the bones and this allows the muscles - attached to the bone - to get maximum leverage and movement range. If you tear a ligament, you have problems with movement (think of sports knee injuries like ACL tears).

Bones are made out of mineral. They would grind together and damage each other without the more flexible and tougher cartilage cap to rub on instead.