Because the strong and healthy can still pass it on to vulnerable people, even if they are not symptomatic. We also don't know everything about this virus yet, as it is new, so it is incredibly dangerous to put the general population at risk by not taking precautions. It could have more detrimental effects on healthy people than we realize. Right now when we don't have a vaccine is all the more reason to keep our distance. A vaccine will be developed eventually, so in the meantime, let's not let it spread exponentially. Yes, the closures are terrible for the economy, but I would rather millions of people not die or have irreversible damage done to their bodies.
You also keep saying that the flu is deadlier, but the coronavirus death toll has only spiked within the last two to three weeks. We don't know that it isn't going to be worse than the flu in a few months time. THIS is why we are being more cautious with coronavirus than with the flu.