Jammysticks
Although you're building hours every time you fly, "hour building" in the traditional sense is generally done in order to meet the requirements (200) of the CPL. There is very little point building hours after you have the CPL, as most airlines aren't too interested in single engine piston hours, so to them a 500 hour CPL is no different to a 200 hour CPL. There are a few exceptions obviously.
Just because someone can fly a plane doesn't mean they'll be any good in a multi pilot jet. That's why airlines generally look for multi crew hours and specifically, time on type. When there aren't enough of these, or when called for by the business model, they will turn to cadet pilots. Quite often the ultimate discriminator isn't hours, but recency, meaning a 200 hour pilot who passed his/her test last month is deemed a safer bet than a 300 hour pilot who passed it 2 years ago.