Why do some people argue that the cenozoic era should not be called the "age of mammals"?

Many people argue that the Cenozoic era should not be called the "Age of Mammals" because there were many other species, such as birds and insects, and many plants were as numerous as mammals were at the time. So they argue the name of the era giving reasons such as these - why should it be named after mammals when there were other animals and birds and plants roaming the Earth as much as they did?