Producing ultra-heavy elements in nature is not easy. So their absence "in nature" does not mean they cannot exist or cannot be created given the right conditions.
Some details:
The valley of stability becomes increasingly n-rich, so neutron capture reactions are essential.
To get beyond lead requires rapid neutron capture in the r-process. The requirements here are a dense flux of neutrons and a capture timescale that is shorter than the beta decay timescale trying to take the nuclei back towards the stability valley.
Once the neutron flux diminishes (these things happen in explosive events like supernovae and neutron star mergers) then beta decay does dominate and takes the nuclei back to the stability valley.
In principle, stable elements of any atomic number could be formed in this way, but ultimately you have to compare neutron capture rates with all the processes that act to destroy the intermediate neutron rich nuclei, such as photodisintegration and fission. For the really heavy nuclei the principle problem is fission - the n-rich heavy nuclei just break up before they can capture any more neutrons given the neutron fluxes that exist in "natural sources".