The oil is running out because there is only so much of it on Earth, and we can only get it out of the rocks so quickly. We use so much of it everyday so it is only a matter of time before there is no more left.
The oil was made up of living creatures and plants from millions of years ago and takes millions of years to form. Since humans started using it in 19th century we have been using it at a faster rate then it is being produced. Its like anything else if you use more of it than is being made you’re going to run out eventually!
Oil is running out because the supply of it in the ground is limited.This is because it takes millions of years for oil to form from ancient living creatures and plants and hence, cannot be replenished in a short period of time. So you can imagine if we keep using oil like there’s no tomorrow, one day, the limited supply will eventually run out!