This can be explained by the fact that because of the COVID-19 pandemic, people spend more time at home.
According to the source, on Sunday morning more than 20.3 million concurrent users were registered on Steam. This is a new service record. There were 6.2 million people in any game at that moment.
The source explains the record for the fact that many people now remain at home because of the coronavirus.
The fact that many users enter the service but do not play is partly due to the fact that they use other platform features, but in most cases this is due to the fact that Steam is logged in by default when the system starts.