True but in boxing you block punches to the body as you do the head, but you don't leave the body wide open in between time.
Why dont their ribs get smashed to pieces by kicks punches and knees?
Why not use the boxing guard? It still stops some power to the head and the body
Well ask yourself that same question. It's obvious that their ribs isn't getting smashed to pieces despite leaving it "open" as you say so that must mean its working. Thais typically have over 300 fights in their career, if open ribs was something to be exploited then surely it would've been along time ago. Thai's also have a rich boxing community and there are many fighters who compete in both so it's obvious they are well aware of a standard boxing guard. If there was a big benefit to using it, then surely they would.
Some reasons I can think of:
1. Clinch. Keeping the elbow tucked in makes it easier for somebody to wrap your head up in a plum. Clinch doesn't exist in other styles, like Dutch Kickboxing where you see them keeping a tight guard similar to a boxing style.
2. Elbows. Similarly, this doesn't exist in other styles and could be a reason for the stance. It's very easy to come around a boxing guard with an elbow and even split a tight boxing guard down the middle.
3. Distance. Muay Thai fights are engaged at a longer distance because any closer and you start to clinch. You never see Thai's touch head-to-head like the Dutch. As a result, you can see the kicks coming and you can block with your knee or forearms.
It's really not that easy to smash a full power shot to a trained Muay Thai fighter. Also, the stance is not really absolute. Just like some boxers don't use a regular guard, there are many Muay Thai fighters who don't adopt the standard stance.