Because modern hybrid seeds give much more productivity, but do not breed true.
Corn variety 1 + Corn variety 2 -> Hybrid with high productivity, disease-resistance, or some other desirable trait (or a combination of desirable traits).
However, the seeds from the hybrid will not be the same as the hybrid. Classically, you'll get some seeds that are more like CV1, some seeds that are like CV2, and some that are like the hybrid (in practice the genetics can be more complicated than that, but that's basically what's going to happen).
If CV1 is resistant to devastating disease D1, and CV2 is resistant to devastating disease D2, the hybrid might be resistant to both D1 and D2. Great! But the seeds from the hybrid are different. 1/4 the seeds from the hybrid will be susceptible to D#1 and 1/4 will be susceptible to D#2, and 1/2 will still be resistant to both. That means that "saving your seeds" = 50% crop loss to disease, if both diseases are present.
Corn variety 1 + Corn variety 2 -> Hybrid with high productivity, disease-resistance, or some other desirable trait (or a combination of desirable traits).
However, the seeds from the hybrid will not be the same as the hybrid. Classically, you'll get some seeds that are more like CV1, some seeds that are like CV2, and some that are like the hybrid (in practice the genetics can be more complicated than that, but that's basically what's going to happen).
If CV1 is resistant to devastating disease D1, and CV2 is resistant to devastating disease D2, the hybrid might be resistant to both D1 and D2. Great! But the seeds from the hybrid are different. 1/4 the seeds from the hybrid will be susceptible to D#1 and 1/4 will be susceptible to D#2, and 1/2 will still be resistant to both. That means that "saving your seeds" = 50% crop loss to disease, if both diseases are present.