I'm wondering why you think supercomputers are actually built for. Besides that, the type of computational problems that are submitted to and that BOINC helps solving are different to those that a supercomputer solves. The former (known as High Throughput Computing, HTC) is concerned mainly -- as the name suggests -- with throughput, while the latter (High Performance Computing, HPC) with speed/latency.

There is a reason why Cloud providers do not yet have "real" HPC offerings (i.e., performance-wise), even though with all the marketing they do to appear otherwise..