It is the ultimate in arrogance and shows total disrespect for you. They know the agreement and that you are waiting and, depending on the situation, getting anxious. They do not give a shit. Perhaps even like it. It is a way of having the attention focused on them.
I once had a "best friend" who was habitually late. He turned out to be totally consumed with himself. I walked away. I always found it amazing that, no matter the circumstances, this type of person always had the 101 excuses for leaving you high and dry, and having no concern for your time, energy, commitment or life. Fuck them!