This is actually a somewhat unlikely scenario. Just as there is 'security theater' there is also 'terrorism theater' - an act of destruction that gains worldwide attention and draws attention the particular cause the terrorist cares about and wants publicity for, be it positive or negative. The essential goal of terrorism is to force people to pay attention by causing something so horrifying that it is guaranteed to get news coverage. This is much more effective against civilians and in peaceful countries, because lots of journalists will show up with cameras - unlike a war zone, where only a few will venture in and where the coverage is likely to vary widely in subject matter and tone. Airplanes and other forms of mass transit are particularly effective because they frighten people enough to create a lot of political pressure, and they're visually spectacular. Likewise bombs in popular or famous locations.

Now a bomb in an airport is much less effective, for a few reasons. Of course, it will kill and injure a lot of people, but it won't create the desired spectacle. Access to the scene is going to be limited by the building's geography and the additional security, so there won't be any strong defining images that capture the essence of the whole event. Bloody and tearful civilians get photographed, of course, but in the collective imagination one group of victims/survivors looks very much like another. People will remember that something bad happened, and where, but it won't be very distinct from similar events that are distant in time or place. Secondly, it would be possible to cut the risk of a repeat incident substantially by requiring advance check-in or use of a credit card to purchase tickets, and prohibiting any entry to the airport facilities proper without a ticket. This is not far away from what already happens with a boarding pass, but is that little bit more traceable and thus raises the barriers to entry. Third, a plane is a confined space with a fairly narrow margin of safe operation. There is nowhere to flee too if a terrorist loses his nerve (and a higher probability of getting caught once the plane lands), and even if the terrorist fails the 'mission' is not a total bust because there will be panic and an emergency landing, which will at least rattle people. In an airport, a hesitant individual may simply give up and leave before blowing himself up, or if his fear betrays him and others become suspicious, he may find himself the lonely occupant of an empty space before being shot or set upon or (worst of all, from the terrorist planner point of view) setting off his bomb and not killing or seriously injuring anyone but himself. Flying is unusual enough for most people, and the awareness of terrorism sufficiently high, that people traveling in airports tend to have elevated levels of alertness.

If the goal of terrorism was simply to kill as many people as possible, then there are abundant opportunities for doing so that are easier than an airport, and arguably more frightening. There are larger and denser crowds of people all over the place where a terrorist could cause carnage with ease. But they lack the visual distinctiveness, the element of captivity, and the mobility that mean not only could you get blown up IN a plane, you could walking along minding your own business and have one fall out of the sky on top of you.

As for the structural considerations, consider that takeoff and landing are the most dangerous parts of a plane flight, on average. Airports are thus more likely to experience a plane-size accident than almost any other location. Planners and architects are well aware of this, and the structures and materials used to build the airport are rated to withstand a higher level of destructive force than normal buildings. An explosion in a massive terminal is likely to result in less damage, not more, because the explosive force will be dissipated in the cavernous space. It's explosions in small confined spaces that are really worrying, because then the pressure wave doesn't have many options about where to go and bounces around within the space reducing everything to small fragments. For the simplest possible demonstration of this, take a tweezers and hold a match in a flame; it flares up and goes out. Now take another match, wrap the head in tinfoil, and repeat. Wear safety glasses.