For what most bodybuilders are after, you pretty much have to operate within a bulking/cutting cycle.
In order to gain muscle mass effectively a bodybuilder (especially one who's been exercising for years) must eat in a surplus and work out efficiently. In doing this, they will also accumulate fat to some degree. This depends on how "clean" their bulk was.
Then in order to lose this excess fat, they have to "cut" by eating fewer calories than they use. In doing this, they also lose a bit of muscle.
If one were to follow the method you propose, they would indeed notice muscle/strength gains as well as likely gaining fat depending on their intake. For most casual gym goers and the general population, this would be a more than acceptable regime.
However many people looking to get into working out research bodybuilding advice and therefore find themselves researching a bulking/cutting cycle. For most people, it's not needed. But for competition bodybuilders or those looking to make very large changes, it is the superior route to go.