Because "springtime" does not have the same meaning as "spring". Besides, the person in this example is obviously asking is spring a season of the year. Spring IS a season. Springtime isn't. It's rather an approximate time during spring.
P.S. I'm not Dutch but wouldn't "Jaargetijde" be a better work for season of the year in Dutch, right? Just like "Seizoen" is more like a "hunting season".