Why did cowboys choose to go on long cattle drives following the civil war?

Those were jobs that were available. There was lots of demand back east in restaurants for beef, the cattle were raised out west far from the few western railroads, so it was necessary to hire cowboys to drive the cattle all the way from the range to the railroad.

Within about 10 years the railroads expanded and went directly to where the cattle were raised and the age of cowboys and long cattle drives came to an end. The cowboys lost their jobs.