EDIT: Thanks for all the responses!
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Protons are positively charged and don’t really like being close to each other, and they have to be really close in a nucleus, so they don’t much “want” to. They only stay together when they are packed just right, with some neutrons to help spread out the repulsion and make it so the short-distance nuclear forces dominate (keep the nucleus together despite a strong push to fly apart). The more that positive charge gets concentrated into a very small volume, the more important the charge repulsion becomes. The nuclear attractive force though, does not grow in the same way (becomes essentially nothing over very short distances so does not add up to the same extent that the repulsion does). Because of this, repulsion starts to dominate in larger nucleii, overpowers the nuclear forces trying to keep the nucleus together.
We don’t exactly “know” if super-large nucleii can be stable (or at least not so unstable that they fall apart almost as fast as they can be made), but the evidence suggests that it is not possible or if possible, requires extraordinarily rare and unique events to happen.
Part of the problem is that it takes energy to force the protons to get so close together that the very short-distance but powerful nuclear attractive forces take hold. Most ways that you can shove protons and neutrons together are not going to match any of the few possible arrangements that could allow nuclear forces to keep things together (the different bits have to be placed “just so” or they pack will not stay together). There will be some randomness involved in any collision that will make a larger atom from adding of smaller atoms together. Most of those arrangements will not be stable, will have weak spots that will allow repulsion to force the packed bits apart.
So, you would need to have a very specific way of all the individual parts (protons and neutrons) arrange themselves in three dimensions in order to keep the nucleus from spontaneously breaking back apart, and it is not clear if any such arrangements actually exist for the super-large atoms. Hard enough just to make the ones that last long enough before breakup that we can observe the larger nucleus and know that we succeeded in making that larger element.
Maybe such super-large atoms could be created in nature, but it appears to require extremely energetic conditions and maybe an extraordinary amount of luck. None of the mass we have in this part of space appears to have every seen such conditions.
With light elements, the number of neutrons is about the same as the number of protons, but as the number of protons gets higher, the number of neutrons required grows even faster. All large elements have several to many more neutrons than protons-like Uranium has 92 protons but about 150 neutrons (more than one isotope so different neutron numbers are possible), so you can imagine how difficult it would be to arrange the protons and neutrons together in just a perfect spacing and organization (as stability would require) if you had, say, 120 protons and 200-plus neutrons. The number of possible arrangements is enormous, yet few (if any at all) will ever be close to stable. Most collisions do not even stick together for even a brief instant, the two (or more) bits just fly apart as soon as they get near each other. Collide and bounce off instead of sticking; they need to stick to make an atom.
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