Consider the social and economical costs of basically putting an entire country on hold. Just that alone on one side of the scales is astronomical.
Quarantining entire infected populations has little evidence base for its effectiveness. Moreover, in a lot of high profiles cases it has shown few benefits.
Consider how many people's health would deteriorate in other ways through not getting the medical treatment, medicine or other help they need during a quarantine.
Imagine starting the spread of the virus through other paths that were previously less likely. As opposed to a home where 1 adult is self isolating and their partner and kids are out all day Monday-|Friday, you might now have one where a family are now contained together for 16 waking hours, increasing the likelihood they transmit the infection amongst themselves, and then after 14 days now have multiple carriers to go spread it through workplaces, schools and the like.
Yes, continue to isolate sick people or those who are likely sick, but don't stop the rest of society contributing. You have to have a pretty damn good reason to quarantine entire populations and you need public support to do it. You can only do it for a limited time too, before the public won't play ball. The transmission rate so far in the UK entirely supports not quarantining currently. It may happen at some point, but there's a massive cost to doing it, so you only do it at the right time.