And yet it makes no sense that a spell capable of healing complex tissue damage and even brain damage cannot mend a bone. Seriously, Mending is a cantrip and does the same thing, it's not a complex task.
Besides, semi-realistic rules are all well and good, but who's the person who is most likely to rack up a broken bone? The melee combatant. Who's the person who is least likely to have the skills or resources to treat a broken bone? Same person. Who suffers the most if you make Cure spells unable to mend broken bones? It's not the healer, I can tell you that.
Now, if you want to maintain some realism while not totally boning the fighter (ahaha), you could rule that you have to heal at least as much damage as the injury that broke the bone dealt before it mends. That way jumping off a building delivers a significant injury (which can be fixed with powerful magic quickly, weaker magic slowly, or natural healing even more slowly), but you don't have to call in the high priest when little Johnny breaks his arm falling out of the apple tree in Old Man Jenkins' yard.*
*Of course, that fall could very well kill little Johnny under normal D&D rules, but if you're going for a semi-realistic setting you've probably got this covered as well.