There's a difference between the understood risk of doing something in everyday life, and using a mode of transport that *may* have a serious safety flaw.
I know drivers can be killed in accidents and I continue to drive. However, if numerous cars the same model that I have started crashing in suspicious circumstances where there looks to be no driver error involved, would I want some assurances that there isn't a critical flaw with my car before I drive it again? Or would I just hop into my potential death-trap car and keep going? I'd stop (in case you're wondering) and would use a different vehicle until mine is confirmed to be safe to drive.
Nobody is saying that the MAX will be grounded forever. However, 2 fatal flights within a few months for a brand new plane is very worrying - especially when it looks as if the flight systems may have intentionally flown the plane into the ground. That's why you ground the planes. When hundreds of lives are at stake, you err on the side of caution until you have proof that the plane is definitely safe.