And why not fix whatever is keeping trees from growing, and then grow trees

I woild guess because that would require you to completely tear up the bitumen and anything underneath it like pipes and wires in order to make room for the roots. Trees are pretty big things y'know and it's not just what's above the surface that matters. You could put a tank like this in say, a train station platform that's raised well above the ground or on a building

Also a tub of algae isn't going to become a health hazard if it gets sick or infested and won't take decades to establish itself