Roblox trying to make themself in a more “realistic” way, this have been proven from their RDC from what i have heard and Roadmap.
Compared to what you said about musicians, inventors and many other, they give their percentage to people to do all the works such as adversiting (if we are talking about musicians, they hire influencers and stuff like that) or quantity of items (if we are talking abt inventors, because no one is capable to make tons of items without any team) .
It also depends when song / item have been showed into public, because each industry has the most profit on its start (UGC is a great example, for example maplestick made a normal head with a black n white gradient, and its one of the most sold items in the marketplace just because it was one of the first made items) although with start, u also should get less percentage from your purchases for paying people who advertise and who does contribute to that process, but over time that percentage should raise for your profit.
But in UGC side, making our cut less overtime, does not have any sense, for what do you give that percentage?? for someone who could just paste 2 line script and spend 10 seconds while making it and they get 10% more that you making UGC, or someone who advertise your work even though u pay from those 30% you get out of all 70%, i dont know for what we give that percentage, it would be more fair if a creator gets 60-70% just like 2D clothing marketplace.
I am former UGC creator, and my collaborator kicked after I quited UGC because ITS LITERALLY not profitable for me, i spend more than 10k just to upload items as limited items, and in exchange i only got around 500 robux, but what could be if there wasnt any Rolimons notifier? same with normal item but they are barely gets any sales (i have burn out because im barely getting 10-20 sales from a 50 robux item and that DOES NOT cover the uploading fee for atleast 1/4.), only exception is when you use an stolen IP as your own, then sure you would get tons of sales from that.
Sorry if i could explain well, i feel sick right now but i wish you got the point that UGC creators cut, is just a steal, for what we are not getting the rest of percentage??