I agree with BaffledCook, as well as a comment Joe made: most people don't keep their knives sharp. A dull straight-edged knife can be dangerous when trying to cut into a hard crusty bread loaf: it will require a lot of force and can easily slide off the domed surface of many breads. (I also don't necessarily want to use some of my more fragile sharp edges on very hard crusts, where I could easily chip or bend the straight edge of the knife.)
A serrated blade can more easily "dig in" to the hard crust of bread. But perhaps even more importantly, if you reach a soft interior, a dull straight-edged knife will simply squash your bread rather than cutting through. (I've been at many people's houses who have dull knives, and cutting bread is a nightmare unless they have a serrated knife.)
I do keep my (straight-edged) knives very sharp. But I have to say that I appreciate a bread knife most when trying to make many even slices of very soft enriched breads, particularly when the loaf is very large and tall. Yes, one can do it with a very sharp straight-edged blade, but the long sawing motion seems easier to guide to me. (It does generate more crumbs, though.) Perhaps I'm just used to it. Or perhaps I've just encountered far too many dull knives at other people's houses who ended up squashing loaves of bread as they cut. You really do need an exceptionally sharp knife to cut through soft bread without potentially squashing the loaf a bit.
Lastly, I'd say the place where bread knives are essential is in cutting hot bread. Every bread baker knows you're not supposed to cut bread right out of the oven, but every person who loves to eat bread usually wants it immediately. When you cut bread that hasn't "set" inside, it tends to leave a gummy residue on your knife as you cut. With a straight-edge blade, this can quickly become a disaster, as your fine edge gets gummed up with residue from the bread and no longer performs as well. The serrations on a bread knife, however, will keep the knife going even through very fresh bread. (On the other hand, hot bread tears easily, so you have the risk of tearing with serrations vs. the risk of squashing the bread with a straight edge. I tend to be more in favor of not squashing bread.)