Why does the notice period of the agreement termination extend for me and SPACEGOATS with the increasing duration of the contractual relationship?

This regulation is based on § 89 of the German Commercial Code.
Its purpose is to protect both parties: The longer a business relationship exists, the bigger is the parties’ trust in the continuance of the contractual relationship and the more worthy of protection they are in terms of termination. The reason is that the business relationships solidify over time and the longer they last, the longer it takes to dissolve them properly.