Because you are giving an OTP to the website, instead of the website giving a push notification to you, OTP mitigates this. It's also just better and way less invasive.
Google lets you use OTP, but only as a back-up option to having a phone and using push notifications. Apparently microsoft as well. Many financial institutions still use SMS for MFA, not wanting to use OTP or app probably because it's "too technical" for older people who comprise the lion's share of investors.
PLEASE, can we all just use OTP already.