People on this forum have been recommending exactly this since the people on the quarantined cruise in Asia returned home.
Because that is how long we have known about the prophylactic effect of hydroxychloroquine.
There were reports of Chloroquine being effective during the first SARS outbreak back in 2003 but it’s likely it was forgotten.
This current Covid-19 virus is actually another SARS virus, 86% the same genome.
Here is a NIH article published in 2005:
“Chloroquine is a potent inhibitor of SARS coronavirus infection and spread”
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1232869/