Look at how the bottom of the spine joins with the pelvis. Question: What changes to that relationship when the weight bearing changes from going through your hips (and ultimately to the ground via your feet) to going through your sit bones into your chair?
Answer: Not much! You'll notice that both the hip joints and the sit bones are below the sacral-iliac joint--which is where the base of the spine, the sacrum, fuses with the pelvis.
As far as your spine is concerned sitting and standing is pretty much the same thing, so why do you sit so differently from the way you stand? One reason is that you may simply not be aware that your sit bones are there to be sat on, or "stood" on. It's common to see people using their coccyx, or tail-bone, to be the point of contact with the chair which causes the pelvis to tip backwards and rounding the lower back, hence the lower back pain many eventually suffer from sitting in this way. You can try pretending to have a tail and always make sure that imaginary "tail" is BEHIND you when you sit.
The surface that you sit on is also a huge contributing factor in how you sit, but as long as it is flat and firm, it will do. My motto is "if you can stand on it, you can sit on it".
This is why sofas are so hard to sit on well. Try standing on your sofa and you'll soon find how hard it is to feel balanced. With your sit bones as your "feet" it is no wonder that sitting on a sofa is hard to do well, and you give up by collapsing into it.
Another reason is that the act of sitting is incorrectly associated with relaxing. Sure, it's more relaxing for the legs and hips, but as far as your torso/spine is concerned, sitting is as equally dynamic as standing. Even the word "relax" seems to be misunderstood. I'm often heard saying during lessons, "That's not relaxing, that's collapsing!" It is perfectly possible to relax into a balanced posture, whether sitting or standing, so that our bones support our weight. Yes, the postural muscles will still be working, but think of them as having tone rather than tension.
How we use our bodies for the mundane things we do in our lives totally and completely affects our pain levels, which in the long term affects how we age.