This is very much what most people don't understand. The primary driver of market stress is uncertainty. Once that is gone and the bad news has been digested, that's it. If lagging indicators further confirm the bad news, that's almost irrelevant because there is no new information content there. Most traders are not stupid, they know the economy is going to tank. The information that there is going to be a recession is already known and pricd in. The question at this point is how much is it going to tank? If it tanks more than expected, or 2nd/3rd order effects materialize (e.g. solvency issues, deleveraging, etc...) then prices will fall more. At this point I'd also like to stress that what you personally expect is not neccessarily what the market expectation is. The market is never wrong in this sence, it is just a consensus of opinions. Opinions are by definition subjective. When new information arrives, the opinions change.
In a mathematical sense, the relationship isn't as much between the absolute level of the economy and prices, it is the tendency of the economy and the prices. You should look at the first (maybe second?) differentials.
Furthermore, values like GDP are inherently lagging. They show data that has happened months ago. There's almost no new information content in that number. Yes the number is bad, but everyone knew it's going to be bad. It sounds like (judging by the market action) that it wasn't worse than consensus.
Now, all the above is viewed through a lens of efficient markets. Reality is again more complex. As it has been pointed out, the price is not just driven by expectations about the economy or stocks, it is very much driven by central bank and government action. Easy money distorts prices. When Tesla which makes a puny 400k cars a year and has just about stopped being loss making is priced higher than VW which makes >6 million cars a year and made $17 billion, something is off. Let's put that into context, Tesla shares have priced in decades of non-stop geometric (20% YoY) sales growth to justify the _present_day_ valuation.
Don't think for a moment that traders don't know this. It's absolutely a game of musical chairs and they know it. Everyone, and I repeat, even the most boring, backwater pension fund board knows that this is a bubble. Everyone is participating and hopes that when the music stops, they'll have already made some safe haven stockpile to weather the storm.