Why do some poor countries have very low covid -19 cases and deaths?

Their medical system is so poor that no one know any death by COVID-19. In addition, that virus is inactive in hot climate. Those countries are very hot and also dry. Without air droplet, that virus cannot stay in air and thus difficult to create infection chains. 

There aren't many people over eighty, or overweight people. 

I still wouldn't go to such places, even though the spreading of it is high in developed parts of the world, in those places you mention, lions walk around with people and could just pick you off if they are hungry.

There's a few factors at play here.  First, they're not a bunch of fat slobs who could be easily herded into a cattle car.  Second, they don't live long enough.  Third, they don't bother with "tests" that are of questionable accuracy.  The truth is over 80% of cases aren't bad enough for the sufferer to notice.

Probably because they have no way of testing their communities and lax reporting too.  If your talking about many African countries, then it's mainly because their usual life expectancy is much lower than elsewhere, so they are countries with few people over the age of 50.

I'd suggest the main reason for it LOOKING like such countries have a low covid cases/deaths is because nobody is keeping records!!   They may benefit from comparatively few people going in and out of the countries.

Yeman and Syria have their own death rates, via other means.

Because like China, they don’t record the true number of cases

Yet Yemenis are more likely to die from an Iranian weapon in Yemen than covid.

More Muslim on Muslim terror.....

The REPORTED cases for sure are low. Not the same thing as "low cases and deaths" though...

They may well have cases but don't have the means to test and keep records on all of the cases or the wish to tell the WHO

Few people go there from outside the country. Chances of being killed by other means are far higher, and with no health service to speak of, even less people make it to old age.