He's the head of the Executive branch. It's his branches job to lead an effort against a national pandemic, just as its the governor's jobs to lead the effort in their state (they're the head of the Excutive branch there too) and mayors within their cities.
So when the guy at the top isn't taking it seriously, that causes a lot of negative affects. It's not that the job has "too much power", it's that its powers, as with the Legislature and the Judiciary, are important as fuck. And such a monumental failure of leadership like there has never been before (to co-opt some of Trump's language) can have disasterous effects.