ETFs are buying so why are you selling?

I don't understand how every other post talks about selling and waiting for a correction to buy back in. If huge financial institutions are still buying why would people be selling their small stack. A correction will inevitably happen but in that time you might be missing out on huge gains. A 20% correction after a 40% increase means you left money on the table.

These institutions didn't get rich by losing money. If 60k is a good price point for them it's good for me.