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This is not surprising considering that about half of a person’s bodyweight is “not-the-person”, but microfauna microorganisms and other passengers.

Half the weight? Hardly.

The human microbiome (bacteria, archaea, fungi, viruses, and other microorganisms living in and on the body) totals roughly 0.2 kg in a reference 70 kg adult — so about 1/350th, or roughly 0.3% of body weight:

A widely cited 2016 study by Sender et al. revised earlier (and often-repeated) estimates dramatically downward. The gut bacterial count is now estimated at ~3.8 × 10¹³ cells, roughly equal to the number of human cells — not "10x more" as was commonly claimed.
The 0.2 kg figure is for bacteria alone, which dominate by mass. Fungi, archaea, and viruses add negligible additional weight.
Most of that mass lives in the colon. The skin, mouth, and other sites contribute relatively little to total mass.
As a fraction of lean body mass (excluding fat), the percentage is slightly higher but still well under 1%.

But notice that the number of gut bacteria cells equals the number of human cells (there's your 50% number). So why don't the bacteria weigh 50% of the total body weight?

Because a human cell is vastly larger than a bacterium. The average human cell is roughly 10–100 micrometers in diameter, while a typical gut bacterium (E. coli, for example) is about 1–2 micrometers long and much narrower. Volume scales with the cube of linear dimensions, so even a modest size difference translates to an enormous volume difference.

A typical human cell has a volume of roughly 1,000–4,000 cubic micrometers (varying enormously by cell type — a red blood cell is small, a muscle fiber is huge). A typical bacterium has a volume closer to 1 cubic micrometer. So a single human cell can be 1,000 to 4,000 times larger by volume than a single bacterium.

Since mass tracks volume fairly closely (both are mostly water), equal numbers of cells can have wildly unequal mass if the individual cells differ in size by three orders of magnitude. There's also a compositional factor: bacteria are mostly water and have very little structural mass per unit volume compared to eukaryotic cells, which contain a nucleus, mitochondria, cytoskeleton, and other dense structures.

So the math works out roughly like this:

~37 trillion human cells × ~1,000–4,000 µm³ each = enormous total volume

~38 trillion bacterial cells × ~1 µm³ each = about 1/1,000th the volume

Which maps cleanly onto the ~0.2 kg bacterial mass versus ~40–50 kg of cellular human tissue. Same count, completely different scale.

And now you know the rest of the story.

This is not surprising considering that about half of a person’s bodyweight is “not-the-person”, but microfauna microorganisms and other passengers.

My doctor wants me to kill off a bunch of em.