A 747 is a much, much easier engineering problem to solve than an equally reliable and resuable orbital vehicle.

Just think of the difference in the range of flight regimes they face: the range of Mach numbers, air densities, temperatures, etc is far, far smaller for a 747. The Concorde, after all, was designed at roughly the same time with the same general goals except for one -- travelling about 2.5 times as fast -- and it was far less efficient, more finicky, and more maintenance intensive than a 747.

Even with a 747, you take significant performance penalties to deal with different regimes and the transitions between them. You could save some weight and maintenance if you got rid of the thrust reversers, the big carbon-fiber brakes, and ground spoilers, which are only used for a few seconds per flight after all, to quickly transition from flight speeds to taxi speeds on the ground.

Better yet, how much more payload could you carry if you could get rid of the landing gear altogether, and anything else that is only used for operations on the ground. Have planes belly-land in gigantic, foam-filled ditches out in the desert. Bus the passengers to and from the plane. Have it take off from some sort of wheeled carriage that it is placed on top of before each flight, and drops just after take off. Of course, you'd have to replace all that high-tech foam for each plane, and all of this will require dozens of person-days per aircraft operation rather than a few man-hours with the current system, but, you could get rid of all that equpment that is just dead weight during flight.

In the case of commercial air travel, those suggestions are crazy -- the performance penalty is small compared to the convienience of having a self-contained, reusable landing system! But add on a heat shield, several different attitude control systems, etc, etc, etc, and soon enough you're in trouble. If everything is resuable and just requires changing out the coffee grounds and air freshener to launch again, you're hauling around so much crap for different flight regimes that you have no payload capacity. Throw stuff overboard when you're done with it, and you're paying for tons of replacement parts. Say we just need to figure out how do all that with stuff that weighs 1/10th of what we know how to build now, and while that's right in the long term, it just begs the question in short run -- we don't know how to do that now.

Anyway, I don't doubt we can do better than we are right now, but it's not an easy problem.