I wouldn't really say "very often". Occasionally, perhaps.

Even from a pure zero-sum mathematical perspective, it can make sense to invest even as much as 2 or 3 months of engineer time on cloud cost savings measures. If the engineer is making $200K, that's a $30000 - $50000 investment. When you see the eye-watering cloud bills many startups have, you would realize that, that investment is peanuts in comparison to the potential savings over the next several years.

And then you also have to keep in mind that, these things are usually not actually zero-sum. The engineer could be new, and working on the efficiency project helps them onboard to your stack. It could be the case that customers are complaining (or could start complaining in the future) about how slow your product is, so you actually improve the product by improving the infrastructure. Or it could just be the very common case that there isn't actually a higher-value thing for that engineer to be working on at that time.