The problem with a resistor is that it obeys Ohm's law. In other words, the amount of voltage drop that it produces is proportional to the current. That might work if you have a steady input voltage and a steady current, but if not, your output voltage is likely to vary widely.
A voltage regulator uses active elements and feedback loops to produce a relatively constant output voltage, regardless of variations in current, and regardless of variations in supply voltage, as long as you stay within the limits of the data sheet. A linear regulator sort of behaves like an automatic adjustable resistor, where the automatic machine watches the output voltage and quickly adjusts the resistance to provide the specified output voltage.
Yeah, the 78xx is good for about an amp, when properly heatsinked. You can use a pass transistor (or several) to provide additional current capacity.
Going from 50V or so to 20 V or so means dropping around 30V. At an amp, that's going to be around 30W. That's a lot of power to get rid of. You're getting into territory where a switching regulator is probably a better fit, though it has its own set of issues to deal with.