It's because there's no need to build it until you need to do this experiment though - this is the sort of bounds-pushing which top-end physics always does (i.e

It's because there's no need to build it until you need to do this experiment though - this is the sort of bounds-pushing which top-end physics always does (i.e. before the LHC was built, sensors as good as ATLAS didn't exist, before LIGO the necessary interferometers didn't exist). What they're proposing to build thought is however a logical refinement of established and demonstrated physical principles - it's entangling the vibrational state of a the big block of Aluminum to a system which you can more easily read out without disturbing it (examples in the literature seem to include circulating currents in super-conducting loops - so you can spectroscopically probe the state of the vibrating thing by looking at shifts in the emission/absorption of your superconductor, which is much easier). Basically it is an experimental setup which is achievable with technology we would want to build anyway as a refinement of existing work.