For arguments sake, let's say they don't. Suppose they can just man up and learn to deal with their first world problems. What do you lose by them taking something to make that easier?
Do you normally fix all of your other problems in the most difficult way?
We all seek ways to get an edge, if not against others, then at least to reclaim some of our own time and well-being.
From their perspective, there are 3 kinds of people. There's the positive, motivated guy who can't wait to get out of bed and go make his mark on the world. Then, there is the similar character who has to take SSRIs or whatever to keep that going. Then there is the lethargic person who has to fight every day to pretend to give a shit, can't keep relationships together, doesn't get promotions, and doesn't feel motivated to learn new skills or hobbies.
The last person doesn't really have the option of being person #1, but they may have a choice between being #2 or #3. Since all three of them end up in the ground after a few more decades anyway, why does it matter to others how they get there? Maybe the last 10 years or so of their life will cause them complications due to longterm use. It's hard to view that as a bad trade for folks who were miserable and/or suicidal for the first 50.