Insects are all not quadrapods, they are not rare, their combined biomass is much greater than mamals

I am not clear on calories - are insects inefficient?

The calorie part is about where you're spending calories. Brain or body? This gets complex real fast though. One easy part to understand is surface area and volume (but there's also things like metabolic rate and many others that go in here). But consider that brains are pretty resource intensive. In a human it's 2% of the mass but 20% of the energy. There's a linear relationship between neurons and calories but humans are an outlier in percent of energy that goes to brains. Additionally, it is the third most resource intensive system (for humans), behind skeletal muscle and the liver[0]. Which those last two parts matter a lot. Skeletal muscle scales with volume. So if you're bigger you have to eat more. The more you have to eat the more time you have to spend eating and your metabolism is probably going to slow. So it gets really complicated here really fast (and I'm admitting to my naivety to the subject matter) but basically it's not hard to see here that if you're pretty big you're probably not going to have the resources (or metabolism) to spend time thinking, which means a downward selection pressure on brain and thinking. There's going to be a Goldilocks zone here (like most things).

So basically if you can get food and don't have to spend significant time thinking to do so, there isn't going to be a selective pressure to encourage brain growth. And if you have a selective pressure to devote energy to the brain you're also probably going to have a selective pressure to reduce energy from elsewhere.

Additionally, I'll add that humans both lost fur and stood up because it is better for thermal regulation (there are other pressures too, there's never _one_ reason). So this also helps with the surface area-volume issue.

Basically what I'm saying is that there's these locally optimal conditions that need to exist at the same time to produce intelligent, tool using, civilization making creatures. (I mentioned in another comment that environment matters too. Can't get a bronze age underwater. But I also didn't mention you can't get a bronze age if you don't have a social structure, something, for example, octopi lack). Basically what astrobiologists are looking for are the combination of these optima necessary to produce certain traits. What many commenters here are missing is the combination part, mostly thinking that only intelligence matters to create a space fairing civilization, but you can't get to space if you can't create advanced composites, explosives, and many other things. Or in other words, the astrobiologists aren't idiots and probably know what they are talking about despite what it looks like. It only looks like they are dumb because people are overly simplifying the problem.

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3046985/

Not disagreeing with you, but aren’t these local minimums/maximums quite dependent on eukaryotic cells, and/or protein-based life? Wouldn’t the resources required by the brain vs muscle be vastly different than what we have know?

Also, what if a non-protein based molecular machinery can be so energy efficient and powerful that species having that can skip some seemingly necessary part of development? But I’m by no means an expert on the topic, just asking. If the reasoning was about absolute limits, I would be much more accepting of them (eg. Energy required by the simplistic work done, like a given skeletal structure picking up a weight with muscles only consuming the minimal physical necessity)

Most of what I've mentioned is more physics based rather than biologically based. Even if your muscles are more efficient your skeleton is going to grow with volume, which means non-linearly. Given this, all those same things will still apply.

Brains are resource-intensive only if you are in a massive hurry. Starfish do fine, entirely without a structure you would call a brain. Packing it all into one noggin is risky, for no benefit. If you're not in a hurry, you don't need nearly as much of it, because it doesn't have to do everything at the same time.

There is no natural speed that life has to operate at. Any aliens we encounter are much more likely to be very, very slow, because it is they who can stand the vast chill void between stars. If they visit, and you dare to chop one down, you wouldn't find anything recognizable as brain inside.