1) The Runes protocol, as far as I know, doesn't provide a mechanism for address blacklists like those used by USDT and USDC in their ERC-20 incarnations. So probably the big, ultra-regulated stablecoin issuers would not want to build a stablecoin with that protocol (this might be actually good, because these blacklists are totally against the censorship resistance paradigm, but many people would surely love to see USDT/USDC on Runes ). Perhaps this will limit stablecoins on Runes to "informal" DAOs like BAMK or to smaller companies from countries without strict stablecoin regulations. Perhaps from Argentina? (there are actually some small stablecoin issuers in ARG, like Num Finance, but I don't know the regulation in detail).
2) Runes also provides no advanced scripting abilities, which would be necessary to create an algorithmic stablecoin.
Whatever the technical hindrances are, whether they'll have or won't have solutions, it still doesn't change the fact that a on-chain stablecoin primitive is needed, and actually more convenient for those users who don't want to send their coins to centralized exchange or to use other blockchains.
Plus I believe an on-chain Lending/Borrowing protocol for Bitcoin through Runes should be built because many RICH Bitcoiners want to OWN Bitcoin, not sell. Who is building this?
As already written currently there are no scripting possibilities on Runes. They are tailored to the BRC-20 "etch -> mint -> speculate" paradigm, more suitable for memecoins. One can probably build such a protocol on top of Runes but that would need an effort as big as the Runes protocol itself.
Perhaps it might be better to create some kind of multi-token API standard for wallets, which can be extended by Runes, but also by older protocols like Omni, and also by newer ones like Taproot Assets / RGB. The standard would basically cover the operations for wallet software and Lightning implementations. In this case, a lending protocol could be done with the most scriptable "base protocol", which in my understanding is RGB.