Quarks and antiquarks can indeed annihilate each other, but in the case of mesons, the annihilation process does not occur because of the way quarks are bound together within the meson.
Mesons are composite particles made up of a quark and an antiquark. They are formed through the strong nuclear force, which is mediated by particles called gluons. The strong force is responsible for holding quarks together within hadrons, such as protons and neutrons, as well as mesons.
In a meson, the quark and antiquark are bound together by the exchange of gluons, which creates a "flux tube" of strong force between them. This tube of strong force prevents the quark and antiquark from coming into direct contact with each other and annihilating.
When a quark and an antiquark come close to each other, the strong force between them increases rapidly. As they move further apart, the strong force decreases. This behavior is called confinement, and it ensures that quarks and antiquarks are always confined within hadrons and cannot exist as free particles in isolation.
In order for quark-antiquark annihilation to occur, the quark and antiquark need to overcome the strong force that binds them together. This would require an input of energy that is typically not available within the confines of a meson. As a result, the quark and antiquark in a meson remain bound together and do not annihilate each other.
It's important to note that quark-antiquark annihilation can occur in other contexts, such as in particle-antiparticle interactions or in high-energy collisions where sufficient energy is available to overcome the strong force.