If I'm going to use my own cup I just bring coffee from home in it.

Personally, when I get a coffee to-go, I want a disposable cup so that I don't have to carry it around for the rest of the day.

Fair enough. I use plenty of disposable stuff in other contexts.

The habitual part is what makes a difference for me with my local coffee shop. I usually walk over around midday when I WFH, mostly to move around and re-energize for the second half of the day. Since I know I will be coming right home after, it is easy for me to adjust my ritual to include the cup.