They've created a product which uses ideas and data that are largely available to everyone. It's not a pharmaceutical-drug type invention.
If they hadnt been bought my MS, their revenues were going to be in the $20/mo subscriptions. And you'd quickly find a lot of people offering $5/mo for 80% of the quality which would be "just fine" for many cases. (Eg., consider copilot).
The valuation of OpenAI is because it was clear a player like MS would buy it, since the marginal economic value of that technology is to a large incumbent who can quickly deploy it "in the places it's needed". It's that ability to deliver "when needed" which enables MS to draw value from it: ie., MS is just capitalising on it's current position with customers.
The AI bit is actually providing a trivial amount of profit over-and-above MS's existing position with customers.