First, AdSense is not always so "meager" for some people.
Second, $10 an hour in the US is pretty low, when it comes to owning or renting a home, paying utility bills, owning a car, paying for gas, paying for things like car insurance, etc. etc.
$10 a hour at 40 hours a week is $400. The government takes a good chunk out of that. I'm guessing that the person who makes $400 may only get to keep about (again, guessing) maybe $330? That would amount to less than $1650 "take-home" pay a month.
A one-bedroom apartment in a not-so-great neighborhood (in Massachusetts anyway) can run around $1000 a month. That leaves the remaining $660 to cover everything else, including food, clothes, other necessities, child care (for some), etc. etc.
Besides the above, if someone can pick up a few hundred dollars a month with something like AdSense - who wouldn't want to? These days people are looking for back-ups to their main income, because a lot of people are worried about being laid off or having their pays cut.