A vulture's stomach runs at a pH of around 1. That is roughly battery acid. It dissolves bone and sterilises the anthrax, botulism and cholera in a rotting carcass on the way down. The bird eats three-day-old roadkill for breakfast and feels nothing, because its gut is a furnace built to handle death.
Here comes the uncomfortable bit for the salad crowd.
Your stomach runs at about 1.5. Not quite vulture grade, but in the same brutal postcode. It is roughly level with a possum and a hawk. Meanwhile a gorilla, an actual dedicated plant-eater, sits far higher, up around 4 to 5, and a chimp, your nearest living cousin, has a gut noticeably gentler than yours.
You did not evolve that acid to break down spinach. Spinach does not fight back. That savage, flesh-melting pH is the calling card of an animal that ate meat, often meat well past its best, and needed to kill whatever was living in it first.
Scientists have a polite phrase for this. They reckon scavenging carcasses mattered far more in human evolution than the tidy nut-and-berry story admits, because only a viciously acidic gut lets you raid a kill and survive the bacteria that come with it.
So you wander about with the stomach of a scavenger and the dietary advice of a rabbit. The stomach was issued by two million years of evolution. The advice was issued by a committee in the seventies.