because the fast neutrons eventually destroy every known material used as the plasma-facing "first" wall. That's something the ITER fanboys are not telling you (for obvious reasons).
That's weird, I've been aware for a decade or more now that ITER is working on assorted possible first-wall technologies and the JET in Culham, England is being repurposed as a wall material testbed. Maybe they didn't tell you but they've been telling everyone else.
The walls are going to be sacrificial, needing to be replaced using remote handling equipment. It's part of the "E" in the acronym "ITER", standing for "Experimental". Lithium, converted into tritiurm and deuterium by neutron bombardment is one possibility for walls as its product is a fuel source for further fusion. Other tougher materials might last longer, possibly decades or more before needing replacement though. ITER is a testbed for such research.