why is 14 day quarantine necessary?

Are three weeks enough to recover from COVID-19?
The CDC survey found that one-third of these adults had not returned to normal health within two to three weeks of testing positive for COVID-19.
Full answer in: www.health.harvard.edu
Does the student need to go through 14 days of quarantine even if the test results for COVID-19 were negative?
To best protect the student and others, have the student stay home for the full 14 days. Check your local health department's website for information about options in your area to possibly shorten this quarantine period. (See When to Quarantine for more information.)
Full answer in: www.cdc.gov
What is the purpose of quarantine during the COVID-19 pandemic?
Quarantine is used to separate someone who might have been exposed to COVID-19 and may develop illness away from other people. Quarantine helps prevent spread of disease that can occur before a person knows they have the virus. CDC recognizes that any quarantine shorter than 14 days balances reduced burden against a small possibility of increasing the spread of the virus. Dec 2, 2020
Full answer in: www.cdc.gov
Why do you have to quarantine for 14 days after travel during the COVID-19 pandemic?
You may have been exposed to COVID-19 on your travels. You may feel well and not have any symptoms, but you can be contagious without symptoms and spread the virus to others. You and your travel companions (including children) pose a risk to your family, friends, and community for 14 days after you travel. May 3, 2021
Full answer in: www.cdc.gov