If we consider a black hole as a relativistic object, then there should be no evaporation, but alas, the theory of relativity does not provide answers to absolutely all questions in modern physics.

Evaporation of black holes is predicted by quantum theory. In a vacuum, particle-antiparticle pairs are constantly being born. If there is no black hole, then the annihilation of particles occurs, and the energy returns to the vacuum and everything becomes as it was. This process is called vacuum polarization or boiling.