Because you probably don't book an averagely priced airfare but as an expert pay way less.
You can do that because of deals and offers i.e. marketing, a tool of the free market, of real competition forcing efficiency and wanting you as a customer to be able to survive.
Trains are still closer to a regulated, less competitive, subsidized market. That is because a rail network requires an investment and planning on often at least national level and often is still in the hands of the respective state or a hundred year old institution. In Germany for example, where flights are often cheaper than trains, while appearing more complex the reality is that DB has the absolute power and the 2-3 real competitors on rail are more there to hide its 21 century monopoly in plain sight. The bigger competitors for DB rail there are buses and planes, way before other private train companies. Too old, inefficient, sluggish and bureaucratic everything to give the passenger fair prices. Don't believe all this counts for cargo, that's a different field.