Leaving your house for 2 hours and the real estate agent doesn’t show up…

I’m amazed at how on a daily basis, agents set 2 hour showing windows, and then blatantly and without any thought for the homeowner, show up an hour after the 2 hour window, or don’t show up at all.
How is it that a real estate agent doesn’t know if they’re showing your house at 3pm or 6:30pm??? Really?
I show houses once or twice a week and use 60-90 minute appointment windows so to minimize the inconvenience to the seller. Some times if I’m running late, I’ll show up right at the end of my window, but if I’m later than that, I call to let the office know.
I can’t understand why this simple process of setting showing appointments is so daunting for so many Realtors. I guess if they don’t know the area well enough and can’t set a route before hand to know what house their showing 1st, 2nd, 3rd, etc. then they are blindly setting times without any idea of what house they are going to first, but in this day of mapquest, GPS, etc. I think it’s pure laziness not to plan your route and set your times accordingly.
Anyone else frustrated by this?