Firstly, it's a myth that covid-19 isn't serious for young people; it may not be as deadly as it is to the elderly but it can still kill.
Secondly, a lot of poor countries have very large populations so there are still tens or hundreds of millions in late middle age or old age.
It doesn't really kill for young, so much so to be statistically insignificant. For example let's use the country I live in.
0-19: 90,000 cases, 0 Deaths 0%
20-29: 190,000 cases, 7 deaths .00003% ~0%
30-39: 127,000 cases, 28 deaths .0002% ~0%
40-49: 124,000 cases, 97 deaths .0007% ~0%
50-59: 110,000 cases, 318 deaths .002%
Now let's take a poor country with so many young like say Niger. 50% of the population is between 0-14, basically 0% chance of death from Corona. 48% of the population is between 14 and 64. Low chance of death for 98% of the population. 2% have a high chance of death, ie: ~450,000 people.
Meanwhile some countries with a lot of older people still need to be vaccinated like Australia, NZ, South Africa, Taiwan, etc.