Why is cartilage replaced by bones in a vertebrate adult?

Bones are complex organs, made of more than just a structural element. Cartilage is just a tissue, usually devoid of nerves and blood vessels.

Chondrichythes live in water (ἰχθύς ichthys is Greek for 'fish'); the difference in specific gravity and buoyancy is considerable, and cartilage is not robust enough to mechanically support your body out of the water.

Vertebrates have evolved to have a bone marrow, which is part of the bone. If the structural element of the bone (the dense mineral matrix) was replaced by cartilage only, then the hollow structure would make things even worse.

Additionally, cartilage has about the same density as water (Pan et al, 2016) whereas dense bone is 60-90% more dense, which means extra weight pulling aquatic animals down.

Short answer, cartilage can't provide the same support as a bone, bone is stronger and stiffer.

Sharks may be big but they have pathetically weak bites for their size and that even when they partially ossify their jaws. They just can't fully ossify the tissue. The main swimming action of "fish" works even with flexible connections because each muscle is linking to the next linearly, the cartilage and collagen is mostly subjected to tension. But things like jaws and limbs need to withstand shear forces and withstand flexion, which cartilage sucks at, but bone is quite good at. It is not coincidence that the first bones evolved in jaw.