Because there's not enough demand to justify the regulatory headache for most types of international payments, unless you have the added benefit of interacting with DeFi protocols during a massive shitcoin bubble.
Also important to emphasize, I know I said it'll be outcompeted by a centralized solution. But actually USDC is centralized because it has a multisig, and all of its contracts are 100% upgradeable. So it's like a very inefficient centralized solution that really doesn't belong on a blockchain or at least not a popular one with high fees. But again, DeFi protocols exist, and people want to swap USDC for crypto hedge funds to frontrun.