Not just effort: there usually are many costs associated with change. Perhaps user disorientation, downtime, as well as a lack of long term understanding of how the change plays out in combination with other factors.

A small improvement may not be worth it right now.

Perhaps it can be deferred. Perhaps it makes sense to bundle it with other changes later. *

* Some changes might reflect better on a product when they are rolled out together; e.g. to signal a major release.