It's not a coincidence at all, once you remember (or learn for the first time) the preferred environment and food sources of the pests you've mentioned. With the exception of the ants, they all prefer a moist habitat rich in organic matter. The physical characteristics of the individual components also come into play, as well.

You are not likely to see slugs taking up housekeeping nor fungus gnat larvae, either.

Occasionally I'll see a centipede or other multi-legged beasties that took refuge under the pot or even in the soil itself, probably because the soil has lots of large air pores, but I never see any infestations of the types of soil insects in various stages of development that are common in heavier soils. Most of those insects thrive in the rotting organic fraction of soils that break down very quickly. Since the gritty mix has a minor organic presence and the organic material has been specifically selected because it decomposes very slowly, the gritty mix is rather inhospitable to that type of soil fauna.

Al

The only thing I have ever seen in mine are, earwigs, spiders, and a couple of ants crawling around on top..

Nothing inside yet though, thank God..:-)

Think about it... Pine bark is pretty much DESIGNED not to be good food for bugs, visible or invisible...