In short, the main reasons for this are:
1) Grains are high in carbohydrates, which get broken down in the body into glucose, causing high blood sugar spikes, high insulin response (to store the glucose as fat), and weight gain and diabetes
2) The government food guidelines are biased towards eating a massive amount of grains per day, for corporate profits, not from any scientific or health basis
3) The genetic modification of wheat and corn in particular, makes them "Roundup Ready", or splicing toxic bacterial genes into corn so the plant produces its own pesticides, which you then eat...
4) The GMO "Roundup Ready" wheat is laced with Roundup (aka glyphosate) which the World Health Organisation rated in early 2016 as a "probable carcinogen" based on many research studies
5) The genetic modification of wheat has led to a higher amount of gluten in wheat, rye and barley grain products. As we cannot break down gluten fully, we need bacteria and yeasts to do this, and bread-making used to be a fermented process to allow these microbes to reduce the gluten in breads and to make them more easily digested by us. This doesn't happen for the most part in modern baking, and high amounts of gluten are causing a LOT of digestive complaints and mental health disorders in particular
6) There are NO nutritional ingredients in grains which are not found in other foods, such as vegetables, nuts and seeds, and other foods. You won't miss anything by not having grains
7) GMO grains, soy, sugar, hormones, antibiotics, and other rubbish, to fatten them up, but somehow all of these things are allegedly healthy for you?!
Wheat, corn and many other grains are linked to diabetes, weight gain, fatigue, sleep issues, depression, neurological disorders (alzheimer's, parkinsons, multiple sclerosis etc), IBS and many other digestive and gut conditions, cancer, and most autoimmune conditions. If you have any of these conditions, you need to consider looking at how grains are possibly causing your health issues.
There are plenty of healthier alternatives to having wheat and corn products, and removing wheat and corn from your diet will improve your health in so many ways.
How have you noticed grains like wheat and corn affect you?