Over-explaining is rarely about clarity. It is about seeking permission from someone who was never going to grant it, in a currency they do not accept.
Notice the second sentence
The first sentence is your answer. The second is usually the apology. "I can't make Thursday" is complete. "I can't make Thursday because work has been so chaotic and I feel awful about it" hands the decision back to the other person.
Try leaving the second sentence out for a week and watch how little collapses.
"A boundary that needs defending was a request. A boundary that simply holds was a decision."
Warmth is not the same as justification
Declining to explain is not coldness. You can be generous, kind, and glad to see someone while still treating your own reasons as sufficient. The people who mind are, reliably, the people the boundary was for.