Why is a long leg cast would still be used today?

For higher levels. Long leg casts are still used when it is necessary to immobilize both ankle and knee joints. An example of this need may arise when dealing with an achilles tendon issue (rupture, lengthening, etc..). If one only immobilizes the ankle, the achilles tendon, which attaches at the heel & originates above the knee joint, may continue to pull, which could be a problem if it needs to be immobilized.