Are you speaking from a position of expertise/research in the area or just making a personal guess?
What about period when people sat 40 hours a week at the desk? It didn't happen either.
To answer your question I think that sitting 8 hours every day is not very healthy, but it's far better than standing 8 hours. Break long period of sitting with some walking, stretching or some light exercise, and you would eliminate almost all negative effects of prolonged sitting.
One part of the 'more healthy' claim I've read about is that you're bound to move a lot more if you're standing. You'll shift weight more. You might wander off for a second, thinking. It comes more natural instead of lifting your rear out of a comfy chair.
Of course it can't be healthy to sit 8 hours straight without some pause for walking or stretching. Combine that with some exercises 2-3 times a week and you would eliminate almost all of the health problems related with sitting.
Why is that obviously bad? I stand for 9-10 hours straight. It took probably two weeks to get up to that point, but I rarely even sit down when taking a lunch break. I don't use a gel pad and I stand barefoot at home, or in some form of barefoot (vivo's to look nice) when I worked in a office. I almost constantly shift my weight. And every 10-20 minutes I do calf raises, stand on one foot, do free squats, something to move some blood. I'm still standing in all of those exercises though and most are just second nature. I stand still while typing, but move quite a bit when I'm reading.