Maybe the industrialized farms aren't saving their old seed (because they're likely all using GMO seed), but the small (family) farms still are (especially organic ones). Why wouldn't you use this method?
Because it's much less efficient for many crops. The technology of saving seeds wasn't unseated by GMOs in the 90s, it was unseated by hybrid crops in the 20s.
"Today, hybrid seed production is predominant in agriculture and home gardening, and is one of the main contributing factors to the dramatic rise in agricultural output during the last half of the 20th century. In the US, the commercial market was launched in the 1920s, with the first hybrid maize. All of the hybrid seeds planted by the farmer will be the same hybrid while the seeds from the hybrids planted will not consistently have the desired characteristics. This is why hybrid seed is constantly repurchased by growers for each planting season."
It just moves to hobbyists / fringe. Mainline agriculture hasn't saved seeds for a long time (coming up on 100 years) for many crops due to the improved performance of hybrids. Seed saving is brought up w/r/t GMOs because it makes for a better narrative. People actually originally attacked hybrids with the same approach ... farmers wouldn't be able to save seeds since their saved seeds would be worse than the hybrids.
Because it's much less efficient for many crops. The technology of saving seeds wasn't unseated by GMOs in the 90s, it was unseated by hybrid crops in the 20s.
"Today, hybrid seed production is predominant in agriculture and home gardening, and is one of the main contributing factors to the dramatic rise in agricultural output during the last half of the 20th century. In the US, the commercial market was launched in the 1920s, with the first hybrid maize. All of the hybrid seeds planted by the farmer will be the same hybrid while the seeds from the hybrids planted will not consistently have the desired characteristics. This is why hybrid seed is constantly repurchased by growers for each planting season."
http://en.wikipedia.org/wiki/Hybrid_seed