Honest question. Did those US people in Afghanistan not realize that they needed to get out earlier?

[Make7]

4. Security Alert: U.S. Embassy Kabul - May 15, 2021

...

The Embassy reminds U.S. citizens that on April 27, 2021, the Department of State ordered the departure from U.S. Embassy Kabul of U.S. government employees whose functions can be performed elsewhere due to increasing violence and threat reports in Kabul. The Travel Advisory for Afghanistan remains Level 4-Do Not Travel due to crime, terrorism, civil unrest, kidnapping, armed conflict, and COVID-19. Commercial flight options from Hamid Karzai International Airport (HKIA) remain available and the U.S. Embassy strongly suggests that U.S. citizens make plans to leave Afghanistan as soon as possible. Given the security conditions and reduced staffing, the Embassy’s ability to assist U.S. citizens in Afghanistan is extremely limited.

...

https://af.usembassy.gov/security-alert-may-15-2021/

It was announced in mid-April that all U.S. troops would be withdrawn by September 11th. There were plenty of signs for months that it was probably time to leave - sooner rather than later.

I think pretty much everyone was expecting the Afghan government to stay in place in Kabul at least until the last of the U.S. troops finally left.