the performance may be about the same, and CapEx as well (or maybe a little higher), but OpEx could be where you make it up in large(r)-scale operations.
And space efficiency is also not to be sneezed at: for some operations DCs/compute can be place anywhere because latency is that big of a deal, but in other places you need to be close to certain things (trading), and real estate can get expensive.
Or provide enough white glove after-sales support and written guarantees to peel away low end mainframe customers at a fraction of the price.