Because cable is a shared RF resource, and your DSL line is (nominally) a point to point link.

It's a completely shared infrastructure from the demarc in your home to the local cable node. It's not very secure and pretty trivial to abuse. Remember this was an infrastructure originally implemented to distribute TV signal.

Because of that TV heritage and the way they grew (on a town by town franchise basis), cable networks were usually a patchwork of really shitty networks up until fairly recently. My (limited) understanding is that on relatively modern cable systems, there is fiber connectivity to the local nodes, and then coax from that device to the homes in the area.

DSL is a switched network of sorts, and provisioning happens on the switch in the CO. Ditto for fiber.