I believe it's because of the taxes.
In many jurisdictions, USDT is considered an "asset" and not a currency. Therefore, converting USDT to ETH is considered an asset-to-asset trade and not a buy-sell transactions.
Forex is a strict term and has a specific tax rate. But in many countries, stablecoins aren't considered money, so they don't fall into this specific category and their tax rate remains in the "grey area".
Is that so? Who's to say stablecoins won't be taxed in the long run? In the US, USD-based stablecoins might be taxed by the government. Not in this administration, but in future ones. Since most stablecoins are centralized, doing such a thing would be a "piece of cake". Even if you use a non-custodial wallet. For decentralized stablecoins, that's another story.
Now imagine privacy-oriented decentralized stablecoins. That would be a game-changer. But now that companies are putting obstacles for developers (like Google requiring KYC on Android developers to prevent sideloading), we should expect the worse. Decentralized stablecoins and even traditional cryptocurrencies might be at risk in the long run. We'll see what happens...