That's one part of the problem, but we also don't have great data in the first place on what diets should be stuck to, especially when it comes to solid evidence of long-term outcomes. We have better data about short-term weight or muscle loss/gain, but whether some diets result over a lifetime in higher or lower rates of heart attacks, or other organ failure, has little solid data, with the exception of a handful of clear carcinogens. A lot of arguments end up extrapolating from a few observed patterns of variation (e.g. what seem to be positive effects of the "Mediterranean diet", which might also be conflated with non-diet lifestyle factors), then attempting to figure out what factors explain those observations.
There has been research showing healthier outcomes from those eating predominantly plant-based diets. That means: less meat than what most Americans have been eating. However, when in 1977, such guidelines were introduced with "less meat", the meat industry pushed back hard [1, see history of DGA '77], which prompted a revised 2nd edition. This revised 2nd edition dropped less meat and instead focused on nutrient composition ("leaner meats, less fats").
So the politics of food science won't let the government publish "less meat, more vegetables" type of research. Instead, it's veiled in micronutrient suggestions. THen people go and buy vitamin supplements to hit the targets, instead of eating the damn dark leafy greens.
