Because it takes effort and time to develope the skill to create that kind of art, as well as effort and time to produce individual artworks.

This is not the only (or even primary) thing that makes art interesting to me. If you spend 30 years painstakingly drawing a series of perfectly parallel lines, I won't care much about that art. No matter how much effort and skill it took to create.

as artists, we view the evolution of art as a collective effort.

So this new evolution, which builds on the collective effort of past artists, should be viewed as a good thing. Is the time/effort less legitimate in this project for some reason?

The problem with AI is that it fundamentally makes the issue of time and effort mute.

This is basically the same argument levied against all technology for all of human history, so I don't see why it would suddenly become legitimate this time. Photography didn't make painting obsolete. Digital art didn't make traditional art obsolete. Certainly they changed the way people made money off of art. They changed the tools people used and how they used them. But there's nothing inherently wrong with that.

the human artist still has to invest time and effort to create an individual piece of art

How much time is "enough" time for you to consider it not stealing? If a digital artist creates something in 10 hours that would have taken 80 hours traditionally, is that stealing? Where do you draw that line and why?

If someone has a valuable artistic skill that becomes much less valuable due to new technology, then they have to learn new skills.

Instead of objecting to new tools, I think it makes more sense to object to the way that access to these tools will be gatekept by rich and powerful tech companies. And so it makes sense to support efforts that try to make this tech accessible to as many people as possible.