It’s a metaphor for the idea that things that are hard to accomplish are often not as rewarding as we might expect.
There is a German song about the opposite. The title, loosely translated, is “the sweetest fruit is eaten only buy the biggest animals” because (as the song explains) the sweetest fruit grows in the highest branches.
This version has only German subtitles, but you might find one with English.
Aren’t the most challenging things the most rewarding?
The most rewarding is to find the exact appropriate amount of challenge
Sometimes we’re told that, and sometimes it’s true, but often it’s an excuse to get people to work harder, eg at a job, but the result is actually lower quality work.
Sounds like sour grapes to me.
Wrathful grapes, even.