Victor, I'm not going to name off three reasons but instead ask you to think what is involved in leg lengthening, and try to explain why it's impossible to do so by the method you refer to.
Many people have heard of biodegradable materials in bone repair and often think it can be easily used as bone replacement. (I'm not implying that's what you think though it might be.) There's ongoing research into this field with better materials being developed and tested in animal models. The different kinds of suitable biodegradable material you ask about isn't used to replace bone but instead they're used as a platform for bone to form. After bone formation has taken place, the material gradually degrades until only the bone manufactured by the person's body remains.
Different materials need to be used depending where the bone repair is needed in the body. You ask about lengthening the limbs (you mentioned a leg in your comments). Biodegradable platform used in bone repair for a leg could be repair of a badly fractured bone where inserting titanium pins wouldn't work with too many shattered fragments. A biodegradable platform would be used that would allow bone to reform after the badly broken pieces are removed. No artificial piece of bone is used and meant to stay in.
The material used needs to be strong enough to be load-bearing but porous enough to allow to allow orthoblasts, bone growth factors, fibroblasts, collagen and tiny capillaries to infiltrate the platform and form solid bone again. I won't go into detail of what particular materials are best or how it actually happens. Suffice to say, this is major surgery and the leg has to be opened up (if not already due to an open fracture. Repair will start quickly but for the newly formed bone to be strong enough for active use, it will take many months.
The surrounding tissue of muscles, ligaments, tendons, blood vessels and nerves are largely intact, although damaged to an extent. They're available to provide the necessary cells and other factors that enable new bone formation. But what you're asking is for the leg to be opened, the bone to be cut through and a piece of bone platform to be inserted. So where will the extra muscles, nerves and blood vessels come from? It's not possible to put the platform in if there's no room for it.
There are already methods that work satisfactory now but they're slow and painful. Leg Lengthening Methods describes a number of them if you want to understand it better.