Why are there only less than 120 elements out there ?
They haven't stopped at 118. The researchers at CERN and Dubna are looking into heavier elements. Such as element 120 and 122 etc. It's all about the "island of stability". https://en.wikipedia.org/wiki/Island_of_stability
Where the right amount of protons and neutrons increase the stability of the nucleus. It's going to be very difficult to get there due to the increased mass of the target nuclei and increased mass of colliding nuclei. So really the only limit to the elements scientists can create, is the tools they use