Hybrid seeds have to be produced every year beacuse:

If the seeds collected from hybrids are sown, certain toxic chemicals starts developing within the seeds.

If the seeds collected from hybrids are sown, the plants in the progeny will segregate and do not maintain hybrid characters.

If the seeds collected from hybrids are sown, the water content of the seed increases every year.

If the seeds collected from hybrids are sown, there is no segregation of characters in the hybrid progeny and the quality of the seeds decreases.