There is a lot more to it than just covering up a seed with dirt. Seeds are different, in size, in habit, in time to germinate, in moisture etc. Larger seeds such as beans and corn are shallow rooted plants. Larger seeds such as squash are deeper rooted. Small seeds such as peppers and tomatos are small seeds and therefore require a different planting depth. Lettuce seeds for instance are tiny and require something way different from beans. And then there are the medium sized hard seeds such as okra that have deep roots but require a different method. If you buy seeds in packets it will tell you on the back an "approximate planting depth". Most often the small seeds should not be planted more than about 1/4". Seeds like lettuce shouldn't be covered at all, just patted down so as to get good soil and seed contact. Large seeded shallow rooted plants like beans should be planted deeper, about 1-1 1/2 deep depending on the soil. But a good rule of thumb is the smaller the seed the shallower it is planted and on larger seeds plant twice as deep as the seed is long.
Why do people plant seeds? Because many times you cannot buy the plants and seeds are a lot cheaer