If someone invites you to dinner, the response of “thanks, but I’m not available” is all that is needed. There is no need to explain that you are busy, or that you aren’t interested or anything else. There certainly isn’t a need to make up an excuse. At some point in our society, people started to think they owed people an explanation for everything.
If you are offered a job and decide not to take it, you should thank the person who offered you the position and then simply say “thanks but no thanks” (hopefully worded nicer than that) but you don’t owe them an explanation for why you aren’t accepting the position. The same applies if you decide the person who has applied for the position isn’t the right person for you. Thank them for applying explain that it’s not going to work out and move on.
People tend to get more offended by the explanation than they do by the answer. In many cases, not only don’t you owe them an explanation, the reality is, they don’t want one either.
To paraphrase Sgt Friday “Just the facts”. The more you elaborate, the more difficulties you create
Have a great day!