First I heard of these exascale computers was when a Chinese group published[1] a pre-print in which they classically replicated the Google quantum supremacy result in about 300s. The Google team estimated that a supercomputer (Summit) would take 10e3 years to do the task, which apparently really got to the classical HPC community.

[1] https://arxiv.org/abs/2110.14502

I'm sure the answer to this is, in brief, "politics". Most government-sponsored use of insane HPC clusters are military, or at the very least, sensitive. The owners of such machines are not famously braggarts.

The really good stuff take 40 to 60 years to declassify.