If the range of energies of cosmic rays is not so far away respect to gamma's, why those are not found commonly in a nuclear reaction?
The range of energies of cosmic rays goes from a few keV (muons at sea level) to 10^20 ev.
The original particles may be alpha particles, protons, neutrons, gamma, neutrinos etc and when they impinge on the atmosphere they create air showers and a number of pions kaons etc are produced which finally decay to muons which compose most of the cosmic rays at the sea level.
The particles are found in all nuclear reactions,or their by products, but their energies are low. Nuclear reactions are of MeV energies and the interactions that can be studied at MeV energies are nuclear interactions and the field is active and called nuclear physics.
Cosmic rays were the original experimental method for studying particle physics interactions before accelerators were built. The largest accelerator built up to now is the Large Hadron Collider which collides protons with energy of 8TeV, ( 10^12) electron volts, orders of magnitude lower than the energies available to air showers in the top of the atmosphere. That is one reason why cosmic ray air showers are still studied intensively, because new physics comes at high energies. The other is cosmological, to study the energy levels of the cosmos.
To recapitulate, even though nuclear reactions can emit the same particles as the ones found in cosmic rays and air showers, the energies available from nuclear reactions do not offer new frontiers in the study of the interactions of elementary particles. That can only happen at high center of mass system energies, the higher the greater the frontier.