Because if a job is automted, it means that the person whose job is automated now is unemployed. Even if we have UBI, it means that the person DOING the automating will get a disproportionate share of the resources compared with the pittance given to the person who was automated.
Personally, I don't want my job to be automated. I write for a living and if AI takes my job, I won't get paid. I prefer to create value in the world that other people appreciate. I don't WANT to sit in a concrete cage (an apartment) and consume media, with no real purpose in society.
Believe it or not, the majority of people in the world need to feel like they are working for something. Yes, some people will be able to find other causes (mine will be the opposition of AI), but others won't. Of course, that will mean the necessity of drugging people with media (and physical substances...why do you think marijuana is becoming legal in more places?).
The end result is a mode of pure consumption for almost all except the elite who control all the production, and they will decide what happens with the world. Personally, I don't want that: I want land and autonomy to use it to grow food and preserve ecosystems. I want the world to be sustainable, and not just set up for the purpose of furthering technology.
You speak of societal changes on a year-scale. I'm talking about decades and the long-term. This level of automation is bad, and won't do any favours for humanity except the ultra-rich, who will eventually perish like everyone else.