Some exchanges do not offer FIAT pairs.
Is that due to regulations or for their own safety to not involve physical money?
Absolutely right! The main reason is regulation. Because when you are using such coins like USDT or similar it it 100% crypto and can't be undergone any regulation etc. But when you add fiat pairs than well-known government organization can come to you and freeze your activity or even close the exchange if they don't like anything. That's why KYC is a must for such exchanges as well.