Biden cabinet members and others in key roles take time to get done. Those people need to be getting up to speed on key issues, and many require clearances. They need to be privy to weekly briefings.

Covid is a major issue. Rolling out the vaccines needs to be carefully coordinated, so there's no lost motion. Virtually all of the states are tapped out on their public health expenditures, and need federal funding for that. An analogy that comes to mind is running a relay race. The next to the last leg is being run, and the runner due to run the last leg will not be let into the stadium to warm up and take his position until after the current leg is finished.