People are different. At one extreme are colleagues who are reluctant to enter into agreements, but when they do are highly reliable. At the other are those who enter into agreements quickly, and with the best of intentions, but are all too often less reliable. Most people are somewhere in the middle.
Follow-up allows people to account for, to accept, to adjust to each other, to the fact that some people are more reliable than others. Or, put more acceptably, some people need to be reminded more often than others that they have obligated themselves to do a specific thing, by a specific time.
It is an art form in the U.S. to follow up in a way which does not imply that the other person is unreliable: in a brief, informal email; with a quick phone call; “bumping into” the colleague in the cafeteria; always mixing a little small with big talk.
America is a nation of immigrants, perhaps all with their own understanding of what makes up an agreement, what it means to enter into, maintain and fulfill one, including how to know if the other party is “still on the ball.“
