I see this kind of thinking a lot. We often focus on the change itself rather than on the systemic response to the change. I used an analogy once to show how I thought about it that went like this, "The only thing Excedrin does is trick your nervous system into not reporting that you have a headache, so it's worthless?" In context people can go from 100% functional to 100% non-functional when gripped with a migrane or severe headache. The meds can mitigate the symptom and they can get stuff done that they wouldn't have gotten done otherwise.
So covering parking lots has a couple of externalizing factors as well. First, it really does mitigate CO2 pollution by replacing fossil fuel power with PV generated power. It also makes customers more willing to spend more time in your store if their car isn't being baked. And finally if it saved you $2M over 25 years, that is $80,000 a year which allows you to hire additional service staff that aren't being paid out of sales margins, and the increased level of service has been shown to both support more sales and higher margins[1].
What I'm trying so illustrate here is that there are additional factors over a straight "saves a bit more than it costs".
[1] There is a wonderful report on how Nordstroms became the most valuable per sq ft store because they focused on service, service, service. I believe it came from the Harvard Business school but it I didn't find it with a quick search of my archived papers.