You need to treat lots of blood continuously, so you have to cut a bypass into the largest, irreplaceable blood vessels, unlike smaller operations like kidney dialysis where you can just "plug into" an arm.
Mechanical pumping of blood is a problem as it damages blood cells. Infection is a severe risk. Any break in the operation or a temporary "plumbing issue" where the machine connects with the body can be fatal. So you need 24/7 monitoring capable of really rapid response.
Red blood cells are meant to travel into capillaries so must deform but will shear easily when subjected to stress which activates clotting action - mechanical pumps still cannot replicate the pumping action of heart.
Heart is truely an amazing organ - don't think we will get close to a mechanical replacement.
Has multiple chambers, Has chambers that pump sequentially with different pressures and volumes, Never needs to stop for additional lubrication, Never needs to be "charged", Never stops for maintenance, Is powered by oxygen delivered by the red blood cells it is pumping, Is controlled by the body's "proprietary" electro-chemical control system, Is not rejected by the body's internal bio-chemical defense mechanisms, Produces only carbon dioxide as waste, delivers that waste
to red blood cells, Doesn't have a single rough or chemically reactive surface, Forms perfect in-line seals around each chamber without crushing red blood cells, Never leaks a single drop, Doesn't retain any fluid in the chambers, Doesn't allow "backward" flow, Has a pump action and pressure that are strong enough to circulate blood up from your toes, but delicate enough to prevent damaging any red blood cells or capillaries
I don't care how amazing the body is (it is), blood flow needs constant flow with a pulse. Put two bellows in parallel, use a linear motor to push out quickly and pull in slowly on each, and voilà mechanical heart without shear.
It's very risky and requires a nurse at the bedside 24/7, so it's a last-ditch sort of intervention.