Many do sell data, actually. I just literally made a post about this, with real figures instead of numbers I've previously pulled out my ass. If you click my profile it should be high up there (otherwise it will be difficult for me to find it in your own instance). This is my instance's version of the post: https://lemm.ee/post/21285233

The main source stated that the "legal" data brokerage industry, that is companies who simply buy and sell user data, was worth $319 billion in 2021.^1

The global data broker market was valued at US$319.030 billion in 2021 and is expected to grow at a CAGR of 7.96% over the forecast period to reach US$545.431 billion in 2028.

Data brokers are the companies that collect the information of users through the internet legally and further, provide this data to various companies

You are right though, some companies (eg Google) simply collect user data and use it for themselves, they don't really sell much of what they have. This means that my figure where your average user is owed $40 per year would be an underestimation, possibly a very large one.

We could maybe add the value of Google to the industry value, however even then Google's value is offset by their various loss-leading ventures - the true value of the data Google holds is completely obscured. In any case, I need to go to bed! XD