Rich people already had their money invested.

To buy the dip you need liquid cash sitting around earning very little, hoping the market goes down.

Reallocate portfolios if that makes sense, but much of the “buy the dip” stuff implies good market timing: To do it in meaningful amounts you’d have to pick the right times to not invest, accumulating cash at a lower rate, then you’d have to know precisely when to invest it again, switching it back into the stock market.