Vegans eat exactly what carnists eat minus the animal products. Since the typical American diet is just eat meat, cheese, and bread, if you subtract out the meat and cheese, you are left with just bread. Eating just bread seems crazy.
Even just "carnist food" is more restrictive than "carnist food plus strychnine". Does that automatically make ingesting strychnine compulsory for a "truly balanced diet"? What matters is if you're getting the nutrients that you need, and plants are more than adequate for that.