They lack calcium, protein and many other vitamins and minerals which birds require. Seeds and nuts are also high in fat, which can lead to liver disease. While birds do eat seeds in the wild, they supplement their diet with many other food sources - nuts, berries, fruit, bugs, etc. Like humans, birds require a balanced diet to remain healthy. Birds enjoy veggies, fruits, pasta, sprouts, grains and even cooked meat such as chicken.
Pellets are also a good source of nutrients.
Seeds and nuts can be given as treats. Cook for your birds. You can find lots of recipes at the Birds n Ways Recipe Xchange . Below are some articles on diet and foods. Some foods, safe for humans, are lethal to birds.