Deadly how? Are they deadly because they are poisonous to touch like the Poison Arrow Tree Frog? Or are they deadly because they actively target and try to kill people to eat them?
How isolated exactly are Ogres just for an example? Well, the Monster Manual tells us "When they establish lairs, they settle near the rural edges of civilized lands, taking advantage of poorly protected livestock, undefedned larders, and unwary farmers." So, here is a single monster that not only actively seeks to live near people, but that is specifically doing so because people DON'T defend their farmlands.
And I can do this for dozens of monsters, where it states that they actively hunt people, that they actively target farmers, that they actively steal from or raid towns. Sure, any one monster species can't be everywhere all the time, but when you are dealing with fifty or more? A hundred?
And some of them are animals, not just humanoids who are competing. You mention the amazon or Africa, but here's a number from Florida. 6,700. That's how many alligators were killed or relocated after being in a residential area in Florida in the year 2017.
Now imagine what would happen if you took 1/60 of that number. That is 111 incidents, but make them Trolls that actively want to kill and eat people.
Or hags. Or any number of other monsters. Again, one set might be rare, but once you start listing everything, it starts stacking up.