Because most people who don't advocate the POV are stopping at the zeroth order analysis, saying that you're a heartless killer if you want to consider any factors instead of just saving lives.
There is no model for any of this, on either side of the issue, and so the off-hand "we need to get back to dating, etc." really does look... heartless. It's just as much a zeroeth-order analysis as the other.
Fixing that would mean giving due consideration to the complexity of the situation, at least attempting to define one's assumptions/model, and being explicit about how much loss of life one is willing to accept.
With inadequate measures, this pandemic could rival the Holocaust in number of deaths. There's absolutely a difference between allowing millions to be killed rather than committing mass murder or genocide, but "let's get back to life as usual" does demand justification.
Just because someone does something I don’t like doesn’t mean I should base my viewpoint around rejecting theirs.