Why will fossil fuels run out one day

Because they are finite resources. We, as a population, are using these resources in an unsustainable* way, meaning we are using up more of the resources than can be provided - it takes thousands of years for fossil fuels to form and we are using them in a shorter time span.
According to many scientists, we have passed 'peak' production for many of these fossil fuels, which means they are in decline.

*sustainability: meeting the needs of the current without compromising the ability of the future generations to meet their needs. So, if we are being unsustainable then we are not doing that.

Hope this makes sense and I hope it helps! :)