I'm really not an expert on folding. I see it like this. Imagine taking everything in your pots and pans drawer out and tying them together with string. What would be the best arrangement for them to fit back in the drawer? The string makes it hard to maneuvere the pans around and the order matters a lot. Solutions to fit them into a drawer would be hard to calculate, but the solution would be that which would fit in the smallest drawer. Proteins are a string of amino acids which can move in many different ways. They form a structure which is in the minimum energy state. To solve it, you guess a structure and calculate the energy. You do this for all possible structures. Nature somehow always picks the structure with the lowest energy. Structures can also be determined experimentally. We can't solve it yet because the calculations take so long using the models we have. Using AI they look at previously determined results to make pretty good guesses for other proteins. To solve it I would guess the AI model needs more, maybe different or additional data. Or mathematical models and calculations could be improved or everything combined. I don't know. It is important because things like curing cancer and killing viruses can be done with a better understanding of how proteins interact with each other of which folding is a part.

There wouldn't be a human, plumber or otherwise, involved. My point is that a robot plumber does not have to do a job the same way as a human one, thus eliminating or simplifying complicated procedures. Also I think your cost comparison is based on how you think the job would be done with today's technology, not how it would be done with the future's.

Why won't the robot go through the pipe and join it from the inside?

Folding is not solved. Alphafold uses results of experimentally determined folds to make predictions of similar proteins. It fails with an arbitrary string of amino acids. It also predicts the final structure rather than how it actually folds. There's alot they are still working on, though.