Why do some people spend their whole lives working as cashiers?

They don't have the skills or experience to do other jobs. They want to keep busy, some thing to get them out of the house, to make some extra money. Or they just like being a cashier. 

I was one for a while, I liked being a cashier. It got boring after a while, so I found a full time job with better pay and benefits. 

I do entry level stuff because my priority is travel. A day's wages from a McDonald's in Japan was enough to pay this month's rent in Mexico.

Given my priorities it's not worth qualifying for a better job. I'd be paying years of my life and tens of thousands of dollars for a job I'd only want for a year or so.

That dynamic might change now with more companies open to remote working. I would be open to doing something more challenging if I could keep the same job and still move all the time according to my own caprices. That would bring down the amount I have to earn though, since I'd not be paying rich country rent even during my saving-years, so I might well end up settling for something just as mediocre wage-wise and still be better off. 

There have been days I've made more than I've spent just from online surveys. A single Populous survey can pay more than a day's accom & food even in a middle income country. 

cause they like doing it

It may serve their purpose .

No one loves being a cashier.  They don't have other options.