Because the wheat is doused regularly with roundup. And consequently the wheat you eat is loaded with roundup. And there are conflicting studies on the longterm safety of roundup. And whether or not it interacts with human metabolic pathways or disrupts those present in the human microbiome.
Animal studies using GMO crops are only conducted for 90 days. Not long enough to see lifetime/lifespan consequences or whether or not there are any intergenerational epigenetic consequences.
Why would you put RoundUp on wheat? It kills 99+% of wheat varieties. RoundUp is used for controlling weeds in the off season using NoTill methods. But never applied to the wheat.