ELI5: Why do all the stocks trace S&P500 average? they’re either all green when spy is green or all red when spy is red

Should i just trade SPY instead? this shit is ridiculous. A stock could have a terrible technical setup and could break support and then just because FAANG is up bigly it gets dragged up too. Was the market different before algorithms ruined it? These are all different stocks they shouldn’t trade TOGETHER.