It's worth remembering that evolution doesn't select for the best as much as it selects against the worst.

The reason we have such sensitivity doesn't have to be particularly game changing as long as it doesn't make us less likely to reproduce.

You can plainly see our big niche adaptations being used everyday. We think good. We recognize patterns. We use tools. We walk a lot, efficiently and upright. We communicate with high precision. We have a surprisingly efficient digestive system.

We're not busting out the ability to smell rain super often, which hints that it might be more in the "doesn't hurt" category instead of being a big advantage.

My guess is that being able to smell disturbed soil is helpful for tracking, either where an animal has run or where something has been buried. Our ancestors were not above digging up a fresh-ish dead animal a canine had buried for later.
But it could just be that rain sense slightly more accurate than looking towards the horizon was as useful then as it is now: vaguely, I guess? It just doesn't hurt anything.