Mostly because it is made in a multistep batch process that requires a few hours in an autoclave to cure the epoxy that bonds the skins to the core. And if you want to make anything but a flat panel you will need a very expensive bond jig tool.

But if you REALLY want spend money like it’s going out of style, try all-welded titanium honeycomb. These guys made a lot of parts for the XB-70 bomber, and the afterburners for Concorde.

https://www.astechep.com/honeycomb