The simple answer is that caffeine and refined sugar are both minor but addictive drugs, particularly in combination.

I think it's about convenience. You can have a cup of reasonable coffee in your hands in just a few minutes.

You can also have even faster at home or at work - and at no cost. It amazes me that people "grab Starbucks on their way to work".

Yes, but that's less convenient. At home, anyway, you have to make it. That's more effort at a time when many are in a rush to begin with. You can have it at work, but many prefer to have some coffee a bit sooner than that and not have to make a trip to the break room.

Convenience is a very powerful motivator, and these people are prioritizing it over minimizing monetary expense.

Think of all the parents taking their kids to soccer, football, whatever. Many of them grab a coffee on the way.

I guess. If it's in the morning, I'll make some coffee at home and put it in a go cup. Otherwise I mostly don't drink coffee during the day unless it's on a long drive and I certainly won't wait in a 10-15 minute line of cars.