The major cloud vendors have their own data centers & do not need to deal with space or power limits at a rack unit. They have the entire data center worth of space and power supply to work with.
10-20 years ago, 1U and blades made a lot of sense because space was a major constraint; now 2U or larger can make sense because 1U has a lot of tradeoffs for size, and power is the bigger constraint.
If space density is still needed, 2 nodes in 2u or similar narrow but tall configurations provide a more square profile that allows generating airflow with larger fans which is more efficient than the 40mm fans needed in 1U systems.