You're right that synthetic graft materials have been studied since the early 1970s. The Food and Drug Administration (FDA) even approved the first one at that time. Since then, various companies have come out with ligament implants.
Everything started out good with a stiff graft that gave the knee the strength it needed. But over time, there were many problems. For example, graft rejection was common as the body saw the implant as a foreign body to get rid of. Inflammation and rupture ended the use of the early ligament replacements.
Later new ligament implants were designed to promote tissue ingrowth. As the body filled in with its own tissue the new ligament was part natural, part manmade (synthetic). The same thing happened with inflammation and graft failure.
Tissue engineering continues to progress as an area of study. Scientists are trying to develop tissues to replace nerve, skin, heart valves, liver, and bladder. Orthopedic surgeons are very interested in tissue that could help with fracture healing. Injury to tendon, cartilage, and ligaments like the ACL would be the next natural step. For now, research remains in the experimental stage and isn't ready for human use yet.