It's actually a requirement for many compliance certifications in the US - previously employed staff are not allowed access to your systems.
If you're in an office - you can wait until you grab the employee and put them in a conference room to fire them before cutting access (as long as security is there to escort them out afterwards). When everyone is remote - that is not an option.
ex: item number 1 for SOC 2 compliance is
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Access controls
—logical and physical restrictions on assets to prevent access by unauthorized personnel.
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When an employee is terminated - they are unauthorized personnel.