Why are so many adults working as cashiers at walmart or sam's?

its kinds messed up that so many adults are working as cashiers... why? i understand that some couldnt afford college and some are olsder and are rttiring,ect, but i mean the americans who were born here, and has the oppurtunities to succeed, why are so many grown ups just cashiers? and then when the young kids apply for the jobs, the managers do not want to hire them because they feel like they are not responsible or whatever... its retarted.. and is it me, or does it seem really hard to find a job in Georgia?