When the tractor is built, they have all these parts integrated together into the "system", and things like pumps and gears and everything in the tractor wear-in when used for the first few hours (burs, sharp edges in general, smut from heat treating that didn't get completely cleaned out, dirt, etc). You swap the filter because it's job is to collect and trap all that junk as it's circulated through the plumbing. Once you've achieved break-in, you change the filter so it's not loaded up with the break-in debris. The oil itself hasn't been worn yet, it's just been transporting the crud your filter has taken out.