I don't understand why any pistols are still being produced without the safety built into the back of the grip.
As an explanation - not an argument - the reason for a lack of grip safeties goes back to 1911 designs being a problem for more than a few people, and an overall design theory of eliminating anything that can go wrong.
On the 1911 thing, many people experienced not being able to fire a 1911 because their hand shape and/or grip in a given situation wasn't disengaging the grip safety. I personally knew a WWII era special operations veteran who explained that they actually taped the grip safeties down because of this. I also personally know a civilian who got into a defensive shooting with his 1911 and got shot before he could get a round off because he didn't grip down enough with a proper grip when trying to shoot. Wilson Combat, Ed Brown, and others began manufacturing beavertail safeties for 1911s as early as the 1970s and 1980s with various designs of humps at the very bottom of the beavertail to help ensure the safety got depressed. So at least with the 1911, there is a long history of the safeties just not getting disengaged reliably for some people in some circumstances.
Similarly, with the 1911, there is an even longer history of people simply not disengaging the thumb safety in the realities of sudden life-or-death combat.
All that said, I've got countless rounds on high-performance 1911s going back to the 1990s, and have never had a problem with any of it personally - but that's me, and I'm not the kind of person who will disregard lessons learned in blood from other people, either. Again, I'm answering your question about why you don't see grip safeties on hardly anything anymore - not arguing for or against them. Just giving the why.
But that why is also behind what Sig Sauer's engineers and Gaston Glock did in eliminating both thumb safeties and grip safeties. The XD's grip safety design looks less a potential problem than the 1911s, for the record. But overall, the dominant trend in handgun design since the 1980s is to get rid of anything that can go wrong in pulling the trigger in a sudden gunfight. Again, not arguing either way, just giving the why.