Figma is a really strong business. You can show spreadsheets and charts to make a pretty convincing case that it'll make a lot of money. It's not terribly risky.
OpenAI is really strong tech without much product market fit. It's got a ton of mindshare, but that's about it. Buying them is making a big bet that you can somehow turn that into a big business.
So one is a low-risk, medium-reward buy, and the other is a high-risk, high-reward buy. Even if OpenAI ultimately changes the world a lot more, its expected value now is close(ish) to Figma's.