Why is it better for people to do nothing than to do menial tasks?

Because a human being is better than bagging groceries. A human being can create, they can entertain, they can get bored and spring up the next silicon valley or find a passion that revolutionizes our society.

A person bagging groceries is less likely to do sometime great. A person bagging groceries for a living can't take a risk and devote hours of their day to something new, because they have to be back at the store to bag groceries every day. If we pay people to do nothing, they'll start doing things. If we pay people to do something, and that something is stupid and not valuable, they will do essentially nothing.

the job of bagging groceries has not changed, and it used to provide a living wage.

Same can be said for the job of waking people up in the morning, before alarm clocks were invented, or the job of repairing televisions and radio sets, or the job of working in a factory, or the job of delivering milk, and so on.

Those jobs don't exist anymore, because we found better ways to do it, and that lets/forces those people do something else. You aren't provided a wage for doing work, you are provided a wage because your work allows something to exist that couldn't have without your work. If I can get a job done without hiring someone, I won't hire someone. When amazon picks up and makes the stores count your groceries automatically, you won't be able to make any wages off of being a cashier anymore, because that job won't exist anymore.

if someone is going to spend eight to ten hours a day performing labor in a market system, they deserve enough to thrive

A person spending 8 to 10 hours a day on a job that isn't valuable anymore is a waste of time, money, and energy.

Our problem, as a society, is that people are still doing these jobs at all. Our problem is that we've accepted that such low wages are acceptable rather than taking the little bit of extra effort required to get rid of the jobs for good. People are doing the jobs of robots, and are treated as is fit for someone in that situation. If you want people's lives to improve, for all people to get a living wage that is good for a reasonable human being, you have to have them doing the work of a human being, not a robot.

Better yet, if a job can pay a living wage, and people have the option of doing nothing instead, that job will start paying a decent amount. Paying people to do nothing is indirectly better at raising the pay of those jobs than forcing jobs to pay a living wage.

Or, you should be happy to pay them to do nothing at all because that's what you have to do if you want a healthy society. Bored people find things to occupy themselves with, we need more bored people, more people with vision and the stable foundation with which to realize that vision.

And if automation and advancement eliminate those jobs, then whatever jobs there are should pay more, not less

They do. That's part of why inequality is skyrocketing. 10 jobs were replaced by 1, and that 1 pays very well. But we still have 10 people who need jobs.

The jobs for humans exist, but there aren't enough of them, so the people working the jobs for robots are getting treated like shit while the "upper" society does just fine. We can't pretend 8 hours of labor is worth shit. It's not about work, it's about production. We must make society encourage production, not work. To say that people must make a living wage from a job that just doesn't create a living wage's worth of goods is akin to saying that in order to get welfare one must go out and move dirt around for an hour or so.

No, just give them the money for free and we'll all be better off in the long run.