There is great stuff on red bubble and I have bought quite a bit...

There is great stuff on red bubble and I have bought quite a bit of stuff there as gifts and so on while I was in lockdown Melb. But I have also bought items that RBL could supply from competing vendors as well. I start with the art that I want and work backwards. IMO most will be shopping like me and look at tshirts or cups or phone cases on different sites and pick the one that appeals the most.
The artists are not silly, and they will chase the best deal for themselves. The fulfilment etc is being done with 3rd parties and so everybody has similar costs. I think shareholders will tend to be biased (I might be if I was an investor) and not look at the competition in an unbiased fashion.