Humans tend to be very visual creatures. We like to be able to see things and are often uncomfortable thinking in the abstract. When confronted with new things, we lean on our experience to interpret new data and decide whether we trust it.
When it comes to the living world around us, diversity is often in the eye of the beholder. Ask anyone on the street to name 10 different organisms and most would probably rattle off a list of mammals. Maybe a couple land plants. But I would bet my life savings* that 99.9% of the responses would be macroscopic organisms and the 0.1% would be someone thinking how much anti-bacterial stuff we use and perhaps put that link together.
And that's how diagrams like this get made (Click to enlargify):
It's a pretty image made by Leonard Eisenberg that was highlighted in a Business Insider article this week, entitled "This awesome graphic of all lifeforms will make you feel tiny". I get the point, which is to emphasize what a small niche we humans exist in over the scale of life. But the absurdly massive emphasis on animals does a disservice to the very intention of the graphic.
I'm not even going to get into the issues of how extinct lineages were drawn or decided in cases outside the animals, I assume there's some "artistic license" in that part. However, what's lost in that unlabeled section where the "Eukaryotes" label was slapped in, dwarfs everything to the right of it. Check out the Tara project data that just came out in Science (summary here, data paper here, all paywalled because Science) where they estimate ~150,000 planktonic species. And that's *just* the ocean. The vast majority of those don't even show up on diagrams like the one above, because we popular science largely ignores their existence, thus the public remains unaware.
It's unfortunate, because the effects of microbial eukaryotes on human health (e.g. malaria), food (e.g. oomycetes and fungal pathogens) and environment (e.g. harmful algal blooms) are enormous.
*Hahahaha, I know, we're not talking high stakes betting here.