Honest no’s are easier to keep track of than fake excuses.

An excuse is a small piece of fiction you then have to maintain. You have to remember which version you gave, keep it consistent, and dodge the follow-up when the same person invites you to the same thing next month. A plain no needs no upkeep at all. Excuses also invite problem solving, because a busy Saturday can be worked around and a not interested cannot.

Try the plain version on the next invite you do not want. I am not up for it, thanks for asking, and nothing after that. Then notice how little actually comes back at you.