Why stop at 118 , Why aren't any more stable elements possible ?
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
The stability of elements is really a question about the stability of their nuclei. What happens with their electrons (and their chemistry) is irrelevant.
Some nuclei are stable and some are not. The unstable ones fall apart by a variety of radioactive mechanisms (alpha emission, beta emission, positron emission...) resulting in different nuclei (all these mechanisms alter the number of protons and neutrons in the nucleus and therefore give a different nucleus as a product). Some decays happen slowly (potassium 40 has a half-life of about a billion years but oganesson (element 118, the heaviest known) has a half life of about 1ms).
The reason why we only see ~118 elements is because we only see the ones stable enough to observe. Anything common in nature would need to have a half-life comparable to the age of the earth (or be produced as a decay product of something else that does).
The reasons why some nuclei are more stable than others is a complicated area of nuclear chemistry or physics. Some broad rules are known. Nuclei with even numbers of nucleons are more likely to be stable. Even numbers of both protons and neutrons are particularly favoured. Some "magic" numbers of nucleons seem to be particularly stable. And, broadly, the bigger the nucleus the higher the ratio of neutrons to protons needs to be to stabilise it. But, at some point, bigger nuclei just get less and less stable so we don't see them (we have only ever made a handful of oganesson atoms).
There might be some magic combinations protons and neutrons that make some super-heavy elements a little more stable, but we haven't found a way to make them yet.
A different type of limit: how far can we extend the Periodic Table? This article, based on a joint study carried out at Massey University (New Zealand) and Michigan State University, suggests that relativistic coupling is seriously degrading the electronic shell structure by the time we get to oganesson (Z=118). We may not see the shell structure survive the next period. The Periodic Table, like the electron shells, would then end in a fog.