why is it that people are still quarantined when a covid lab test is available?

It's not known how long it takes for a test to pick up an infection after you've been infected.

 

Imagine you became infected by breathing it in on the plane journey to Thailand, it's not going to show up until some time later.

 

This is the unknown so they apply a 14 day quarantine as you are highly likely to develop symptoms and / or test positive inside those 14 days.