Why fear god’s voice?

Exodus20:19 says, “And they said unto Moses, Speak thou with us, and we will hear: but let not God speak with us, lest we die.”  Why do Moses’ people fear gods voice?  Exodus 20:22 says, “And the LORD said unto Moses, Thus thou shalt say unto the children of Israel, Ye have seen that I have talked with you from heaven.” We have seen that god did speak to them and none have died yet.  Moses has spoken to him a lot of times and he is still around.