They are Arteries and not veins which are usually harvested from your
leg. Arteries are much stronger than
veins and can tolerate the pounding much more. My bypass failed after 3 months, two veins closed up. However, the LIMA (artery) is still fully open.
In the UK, when a bypass is done on the left main artery, it is usual practice to graft 2 vessels, one is really a backup. If you have 2 arteries grafted to this vessel, rather than one with a vein, it will last much much longer.