What's wrong with those machines? They're off-the-shelf machines that can be manufactured in large number as necessary and they're subjects to engineering improvement over time.
There are certain disadvantages to artificial mechanical machines.
1. Body's rejection to metals/plastic made objects. Patients who use those devices must take medicines to suppress their immune system to keep those machines functioning inside their bodies.
2. Mechanical machines are also very expensive because they are built rather than 'grown'.
3. They have moving parts which make them less reliable.
4. Many of those devices need external source of power instead of using body's energy (unlike biological machines that are highly 'plug and play') which makes them harder to maintain. It also increases costs because patients have to routinely visit hospitals to replace the batteries or fix them.
5. Even if you build highly advanced mechanical heart with non-biologial materials, it could have magnetic interferences and software level problems which are non-existent with biological machines.