What is the harm in routinely pulling your entire portfolio in and out of the market?

Hypothetically, if there are signs of uncertainty or a dip coming up, why not just sell your entire Schwab or Robin Hood portfolio, sit in the sidelines with cash, and then jump back in. So, you kept (hypothetically again) $500k whole instead of taking a loss. I understand you can’t always time it perfectly, but what is the downside from doing that from time to time?