Good question, but it's not the case always. People earn very well via adsense & sometimes yes people miss out earning good money or experience fall in their revenue via adsense.
In-fact all the clicks on the ads are not calculated equally. Let me give you an example, let suppose you have a travel blog and you run advertisement there. Now there could be an ad of domestic airline or another case when the ad of international airline can run on the ad position.
Considering international flight would be costly, and its advertiser is ready to pay better commision or CPC then the Domestic flight advertiser because he is earning less.
This is something similar to targeting to higher value keywords and accordingly getting higher return vs ranking for low CPC keywords.
Opportunity is there for everyone, some does exceptionally well & someone gets missed out. Also there could be a couple of other reason too like invalid clicks (you yourself clicking on ads on your website), improper positioning on the website, targeting issue as I explained above, crawling issues etc.
I hope this was helpful.
All the Best,