Let's say the 4090ti is priced around $2500. 16 of them would set you back just $40 grand. That's quite a bit for the average consumer, but people spend that kinda dough on cars/boats/planes etc. It's well within the realm of dedicated hobbyists. For companies it's nothing.

Someone will come along selling pre-builts for this purpose, if that's not already available for their current research/enterprise cards.