No gratitude

Caller: "So that's all fixed now then?"

Me: "That's correct sir"

C: "OK, I'll call you back if I have any more problems, bye"

M: "Thank you"

This is the final dialogue of 75% of the calls that I take in a day.
Do these people have so little faith in the support system that they believe that they do not need to provide a 'thank you' as they will surely be making another call soon enough because either my support was second rate or there will be another issue that they will have to call the helpdesk for?

Golden rule, thank your helpdesk, they honestly do remember it. Even in a corporation with 12000+ employees, I can vouch for that.