...Or to make the effect more obvious: extend your arm from shoulder to finger tips into a straight line. Again rest your other forearm on top of the fingers. Now try to lift your other forearm by merely extending that unit at the shoulder. A lot of work for the shoulder.

Try it again but this time lifting with all the other joints to (bending the elbow joint, extending the fingers and wrist).

Or to make this even more obvious (with two different lever arms, but different amount of muscle participation):

Start with the elbow joint entirely bent, so you can rest your other arm directly on top of the elbow. Now lift that arm by merely lifting the elbow up with the shoulder.

Extend that arm out again (elbow joint no longer bent) so you now have a huge lever again, and rest your other arm on top of the wrist joint. Now lift at the shoulder AND bend the elbow joint. It's still easier.

... The power of muscles don't really "add together".
In a chain of muscles, you're really only as strong as the weakest muscle. If you have a couple of strong muscles and one weak one, that weak muscle is going to collapse under the force of the stronger muscles.
... But still, all they (calves) ever did was transfer the force of my quads & glutes.