It's cheaper to build undecorated boxes. Calling it high art is just a cover. There is also a skill shortage since nobody has paid for quality in generations. The craftsmen that meticulously laid intricate art deco brickwork don't exist anymore and no modern mason can seem to do as good a job. Instead we get uninspired generic concrete and glass prisms designed to maximize square footage for the landlords.
I imagine (but I have no real information on this, so I might be wrong) that cathedrals were always works in progress, on which you could throw a small percentage of your surplus in the good years, and keep idle in the bad ones (or maybe also a bit the opposite, in anti-cyclic fashion). See how the Sagrada Familia is being built in Barcelona (and has been, for 137 years): with an annual construction and maintenance budget of around 25 million, a very small yearly amount for such a huge project.
