美国为什么不执行强制国际旅行必须的隔离措施？

台湾做了，所以疫情几乎没有了

All inbound travelers cannot quarantine at home and must stay at a quarantine hotel to undergo home quarantine if other persons who didn't travel with them also live in the residence. Even if there is a separate room (including a separate bathroom) in the residence, they should undergo quarantine at a quarantine hotel.

韩国做了，所以疫情几乎没有了

All visitors to Korea are subject to a 14-day mandatory quarantine as described in the “Quarantine Information” section below. All arriving passengers are required to download and respond to daily questions via a Self-Diagnosis Mobile App for 14 days. See: http://ncov.mohw.go.kr/selfcheck/.Apr 8, 2021

US CDC guideline for international travel:

If you don't get tested, stay home and self-quarantine for 10 days after travel. Avoid being around people who are at increased risk for severe illness for 14 days, whether you get tested or not.Apr 2, 2021