It's not about drawing dicks on a wall, it's about paying 1 satoshi for each pixel, and being able to rebuy them instantly. It's a different market mechanism than just buying pixels
And why can't BCH do that on-chain? Fees too high?
The guys that will fight for this advertising space will buy again and again the same pixels, maybe 500 times per second the same pixel. Bch can't properly order as many transactions
There are plenty of ways to avoid that problem which does not exist. It will be a long while before BCH is doing 500tx/s, let alone a single pixel, please avoid logical fallacies.
Transactions in bch get confirmed in about a second. That's too much for 500tx/s if each tx needs to be sure the previous one was confirmed (which is the case here but not everywhere)
Moreover blockchains (bch included) can't do those things where one action is done because of another : games, satoshis.place, ect .. (Even with the first seen first safe mechanism)
If you but a pixel in this "place", you do it because you've lost, you don't want to buy it for free. You need to be 100% sure a specific action happened and this can't be done with
"you do it because you've lost, you don't want to buy it for free" -> because you've lost it, and don't want to buy it if you already have it. So it depends if someone just bought it
a blockchain (this is why Hashgraph claim to be "safe") But I've not read satoshis.place code so I might be wrong and it might be slow enough to be be done on a blockchain