Why are humans overpopulated? Are we not supposed to be here or something? ?

In the last few millenia humans have been on the top of the food chain, and with industrialization and the better development of medicine, our numbers have increased a considerable rate. 

Some cultures limit or discourage access to birth control because they consider it to be morally, religiously, or politically undesirable. About 222 million women who want to avoid pregnancy in developing countries are not using a modern birth control method.

the earth is overpopulated

It's not so much that there are too many humans. The real problem is that the relatively rich people consume a lot more than their fair share of what could grow back.  Blaming "the third world" is wrong/hypocrite, because the average Japanese or USAmerican consumes 300 times as much resources as the average Ethiopian.

The earth cannot carry billions of car-owning tobacco-smoking fashion-following carnivore humans.

and believe it or not, we have invented ways of shagging eachother without making babies. Things like condoms, IUD's, "the pill", vasectomies and tubal ligations

There are so many of us because

1. agriculture 

2. invention of artificial fertilizers

3. advances in medicine stopped many diseases reduced infant and adult mortality. 

4. antibiotics prevented many deaths

Life expectancy used to be around 32 years  Now it is over 70 years. Fewer deaths than births means a population that will keep on increasing.

This is the mentality of people in undeveloped and  third world countries - they look around and think wow there is hardly enough food to feed the people that are already here, people are dying in the streets, how am I going to survive past age 40 and into old age? Answer - I will have 12 children so if half of them die from famine or disease or war I will still have 6 children to work and support me.

Problem solved.

We are not.

They cram people into cities