8 years is not enough time for a President to get things done. As soon as they get into office, they tend to focus on the next election or short term goals, diminishing their ability to govern. This results in the administrative state taking more of the responsibility of executive actions.
Imagine if a CEO of a major company was changed every 4 years. They need to worry about short term solutions, instead of long term as they only have a limited time running the company. It wouldn’t run efficiently.
FDR was able to dramatically change the government in his 13 years in office. He essentially set up the current system of government we have today. If he only had 8 years, I don’t think he would have had enough time. I’m not saying FDR had good policies, but just using him as an example.
The argument could be made to change elections from every 4 years to 8 years as well, but that may be harder to sway in my opinion.