Well, most companies aren't going to pay for a standing desk (and we're forbidden from using our own equipment). There's also the cultural thing: if I was standing I'd be able to see across the whole floor of cubicles which not only makes me very conspicuous but might also make other quite uneasy (like I'm watching them).

Anyway, that's one of the reasons why I'm leaving so I can work from home at my standing desk. I believe standing desks will be the next trend in perks that great companies offer - so it'll probably take >15yrs before it becomes mainstream.

Desks that don't support standing. Workplaces which frown upon making your own standing desk. Commutes where there's no "not sitting" alternative. Professional lunches/dinners. Meetings. There are plenty of reasons people need to sit. A half-hour commute each way, one 1-hour meeting, and lunch with coworkers will put you at your 3-hour sitting time per day without anything else.

It's hard for a lot of employees who don't have a manager on board. I think that's getting easier though, as standing it catching on.

It's also hard for some people to afford a good standing work station. I just use a bookshelf, but it depends on your computer.