When it comes to doing gross, dangerous, repetitive, manual labor automation does make sense.
That is not the issue.
The issue that many have is the possibility that the automation will take away all the valuable labor that people do to provide for themselves and their families.
Even _IF_ the Capitalist elite that will inevitably end up owning all the AIs and the automatons run by it provide UBI for the people that _they choose_ to maintain as 'stock' those people's lives will be essentially meaningless.
Do you expect those peons sitting in their peon living cubicles, eating their peon quality food, listening and watching entertainment equivalent to the echoes of a fart in a jar to have any encouragement to strive to be more?
If there is no real opportunities to try, fail, grow, and compete and collaborate with others in pursuit of passion, power, prestige, wealth, stability, all the things that make life both wonderful and torturous then what you get is the ennui and malaise that so many young people now have as they look towards the future you are fanboi-ing.
If you think the birth rates in First World countries is low now, just wait until all but the the children of the 1% see absolutely no reason to try to live because automation and AI took away every opportunity worth Capitalising.