Why is so much invested in AI but not in fusion power?

Probably because AI appears to work, more or less, and now it's just a race to make it better and to monetize it.

Before ChatGPT, I'd guess that the amounts of money poured in both of these things were about the same.

Imagine getting the opportunity to sell microsoft office to the entire world again how much money is on the table. It doesn't even matter if it works. If you can get the mindless corporate buyers to purchase it along with all the other useless redundant junk they also purchase you are making money hand over fist.

Fusion power on the other hand has to work as it doesn't make money until it does. You can't sell futures to people on a fusion technology today that you haven't yet built.

There is probably not any large market for fusion power as we conceive of it today.

You will get a different result if you revolutionize some related area (like making an extremely capably superconductor), or if you open up some market that can't use the cheapest alternatives (like deep space asteroid mining). But neither of those options can go together with "oh, and we will achieve energy positive fusion" in a startup business plan.

Because wind, solar and battery tech have given us most of the benefits of fusion power and it actually works today.

Bad comparison.

Investment in fusion is huge and rising. ITER's total cost alone will be around $20b. And then there's Commonwealth Fusion, Helion, TAE and about a dozen others. Tens of billions are going into those efforts too.

There are a lot of areas that could use more investment but aren't getting it. The way this works is complicated. The best explanation comes from really understanding Moore's Law. The main effect of the law was really about investment, about securing investment into semiconductor fabs rather than anywhere else.

See, every fab costs double what the previous generation did (current ones run roughly 20 gigadollars per factory). And you need to build a new fab every couple of years. But, if you can keep your order book full, you can make a profit on that fab- you can get good ROI on the investment and pay the money people back nicely. But you need to go to the markets to raise money for that next generation fab because it costs twice what your previous generation did and you didn't get that much free cash from your previous generation. And the money men wouldn't want to give it to you, of course. But thanks to Moore's Law you can pitch it as inevitable, if you don't borrow the money to build the new fab, then your competitors will. And so they would give you the money for the new fab because it says right on this paper that in another two years the transistors will double.

Right now, that "it's inevitable, our competitors will get there if we don't" argument works on VCs if you are pitching LLM's or LLM based things. And it doesn't work as well if you are pitching battery technology, fusion power, or other areas. And that's why the investments are going to AI.

Because the money is in software.