You've haven't touched or seen a 1:18 figure in 20 years.
Almost every 25th and on GI Joe could bend their knees to almost touch their heels to their butts.
Elbows were ~90°, but a lot of modern lines can do better. Some HAsbro figures even had double jointed elbows so that they could almost touch their own shoulders.
Nothing needs to be fragile in 1:18 and can support pretty much every joint a 1:12 figure can.