If I suddenly inherited $1m in cash and I planned out my desired portfolio allocation, I would want to enter into it over the course of at least a couple months because:
1) Interest rates are so high, you'd still be doing well earning 5% interest with the money parked in money market / bond / treasury ETFs. This significantly reduces the opportunity cost of waiting to invest the money or entering into a position slowly over time.
2) Markets fluctuate on a day to day basis often in response to things like fed meetings, jobs reports, earnings announcements, etc. You can average out the volatility by entering into the position over a few months (which, again, due to high interest on extremely low-risk cash/treasuries/bonds returning ~5% currently, the opportunity cost is minimal).