I've been trading stocks for a month now and there is one fundamental thing I still don't understand.

Why did everyone agree to buy stocks in a company that does well and sell stocks in a company that does poorly? It seems that the entire market is running on this arbitrary agreement meanwhile theoretically the performance of a company shouldn't affect the stock price and if let's say Micron was a completely made up fictional company with completely made up revenue then trading stocks would be no different? I'm struggling to formulate my thought exactly but I hope you understand what I mean. Sorry for a regarded question.