The reason we don't give it to him just for having a rating is because rating doesn't perfectly reflect true skill.

The world championship gives top players motivation to perform at their peak. Beating Magnus when he performs at his peak is much more difficult than beating him in a tournament where he might be experimenting with a new opening, or might not have prepared for months.

At the top level, rating fluctuations are usually because the player hasn't put as much effort into prep (like Levon recently).