Note that in MN 137 there are two types of equanimity, followed by a third state where the process of fabrication is abandoned altogether.
"There is equanimity coming from multiplicity, dependent on multiplicity; and there is equanimity coming from singleness, dependent on singleness.
"And what is equanimity coming from multiplicity, dependent on multiplicity? There is equanimity with regard to forms, equanimity with regard to sounds…smells…tastes…tactile sensations [& ideas: this word appears in one of the recensions]. This is equanimity coming from multiplicity, dependent on multiplicity.”
This process involves first constructing then dismantling one form before moving on to another, as the practice progresses. So equanimity is a fabricated feeling. In fact the only difference between equanimity and the fabrications of exertion is that it requires much less effort (Thanissaro), and therein lies an allure.
The following shows that equanimity is indeed a developed factor, but that work is only possible when the mind is in a particular state.
First the sutta explains that when the mind is restless that is not the time to develop investigation, energy and joy. Then it goes on:
“Now, on occasions when the mind is restless, that is the right time to develop calm as a factor for awakening, concentration as a factor for awakening, equanimity as a factor for awakening. Why is that? The restless mind is easy to still with those mental qualities. Just as if a man, wanting to put out a large fire, were to place wet grass in it, wet cow dung, & wet sticks; were to give it a spray of water and smother it with dust. Is it possible that he would put it out?”
“Yes, lord.”
"In the same way, monks, when the mind is restless, that is the right time to develop calm as a factor for awakening, concentration as a factor for awakening, equanimity as a factor for awakening. Why is that? The restless mind is easy to still with those mental qualities.”—SN 46.53
Most practitioners would recognize the equanimity that comes from multiplicity.