Because thermal energies are far lower than what those particles are depositing. The thermal movement at those energies would still be ionizing because it would

Because thermal energies are far lower than what those particles are depositing. The thermal movement at those energies would still be ionizing because it would some nucleus away leaving a few stray electrons in the lattice. But of course, sooner or later the secondary, tertiary, ... particles will end up as thermal energy. And of course any ionization always has a thermal component, the lattice will always get a transfer of momentum.