rod03801: People don't get better over time necessarely. But whether they get better (or worse) isn't the point. Ratings are used to give an estimate of ones true strength. The argument to use recent activity is if a rating is based on recently played games it's likely to give a better estimate of ones true rating than if it's based on games played 5 years ago. Just as a rating based on many games is likely to give a better estimate than a rating based on a few number of games.

A rating system like BKR uses some measurement to determine how likely the rating reflects your true strength. The more likely (according to the measurement) the rating reflects your true strength, the lesser your rating changes after a result. The more likely your opponents rating reflects his true strength, the more your rating changes after a result. In BKR (but also with ELO), the only thing considered for this measurement is the number of games played. Other rating systems, like Glicko based systems, also consider recent activity. The more recent you played your games, the more likely your rating reflects your true strength, and the lesser your rating will change after a result.

Here's an example why recent activity matters. Suppose Bobby Fischer was still alive. Would you consider him stronger than Anand? Fischer played his last official games in 1972, giving him a rating of 2785 - a rating he would still have under BKR rules. On the most recent FIDE list, Anand has a rating of 2783. Personally, I'd consider the 2783 which is (partially) based on games played this year to be much more relevant than a 2785 rating based on games played way back.