Because of the past.
I heard it was someone at Ford that took the gamble to go all out and purchase heaps of Rh to combat the rise in Platinum and Palladium and that's what sent it to $10K.
Look at Palladium rising again, and see how it is affecting Rh, it's like history is repeating itself.
Palladium eventually cooled off and they ended up writing off something like $1billion!!
So I doubt that they are going to make that mistake again and are wary about putting in big buys.
There must have been a surplus around the price dropped like lead, but reports have it that the mines in SA have cut back on production and we might be back in the era of lack of supply.
Like you say, it would make sense to buy it by the lorry load at the levels it has been.
But it looks like they are starting to take notice and putting in some buys again to hedge against the risk of it getting out of control again.
To answer the question about the spread, try looking at the historical bidding price of Rh when it was at $10K. You will find that it wasn't anything like it is at the lower levels.
Plus you could also invest in the XRH0:LN which doesn't have much of a spread.
https://www.bloomberg.com/quote/XRH0:LN