The whole ideology of capitalist society is that you're supposed to be able to pursue whatever goal you'd like, that you accept the consequences of that decision, and that there is no common good by which individuals should be directed, at least by force. Now I don't think that's the case, obviously, but that's the ideological line. Thus the creation of an "army" of structural engineers would involve some level of coercion to direct the wants and needs and mold ordinary members of society into the engineers of that society. The precondition for this is only being a member of society with sufficient funds (or ability to loan) to enter the education system. But the precondition being 'molded' this way is different from the army system in most Western countries in which there is no conscription. You never chose to be molded that way, you were simply born with access to the education system. By joining the army you chose to be molded that way. I concede that 'molding' happens all the time from every imaginable source, but it seems to me to run directly counter to at least the
spirit of freedom to pursue one's own ends under such influence. The invisible hand is superseded by a government's education secretary.
With regards to your point, the answer probably lies along the different types of reason. To put it crudely, if we were to ask whether a bomb made by a mad scientist is 'good', an engineer will first be directed to look at it in terms of destructive capacity, wear/tear, whether it can be used in the rain, and whether the production line is future-proof against enemies. The philosophy student, hopefully, will be directed to ask whether it was ethical for the mad scientist to create the bomb. The difference here turns on the meaning of the criterion (goodness), which is viewed from two different perspectives.
Plenty of "liberal arts" degree holders really do engage in analysis of complex, real world problems. For instance, the debate around, say, pornography (a very contentious topic which can and does inform legislation), distributive justice, or taxation (in the more abstract sense) is usually held by philosophers and sociologists. These are extremely important and complex problems that are informed by empirical data, not unlike in engineering. It seems as though you expect the philosopher to come forward with a ready-made solution, and indeed many do, but the field operates differently, it operates through dialogue, not only learning from past work. Next, it is worth considering whether working on the real world problems "close to you" is more important than more abstract ones. Philosophy and sociology work at a slower pace than the already slow engineering. It takes years or decades for new engineering works to make their way to pervade society (not merely to influence a few companies in tight competition with another) and it likewise takes a long time for attitudes to change.