Because they give too much of it away for free? Most casual use fits into the very generous free tier.
This is very similar to Uber which lost money for 14 years before becoming profitable, but with significantly more upside.
Investors see the growth, user stickiness and potential for the tech; and are throwing money to burn to be part of the winning team, which will turn on the money switch on that userbase down the line.
The biggest companies and investors in the planet aren't all bad at business.
You can if you're still growing. ChatGPT is the 5th most visited site on the planet yes, but it is still growing hundreds of millions of visits with every passing month.
They aren't turning a profit because they aren't monetizing the vast majority of subscribers in any way (not even ads). LLM inference is cheap enough for ads to be viable.