Part of that is inertia, since humans have been using selective breeding to accumulate beneficial mutations for millennia, and they only rarely mess up and turn plants that were safe to eat into unsafe ones [1]. Because of that, conventional breeders don't see the point in doing strict tests they haven't been doing before.
But in the end, that different treatment just leads to attempts at making large genomic changes without having to report them as genetic modification (e.g. using radiation to induce mutations) that are probably less safe than relatively targeted methods like CRISPR.
[1] the best-known example is probably the Lenape potato https://boingboing.net/2013/03/25/the-case-of-the-poison-pot...