I wish I had that at least once, to know how it is like.

It's really fascinating in retrospect. The intense fear can be upsetting, but if you're used to nightmares then you'll be fine. I've always loved terrifying/surrealist art/media, so after I wake up it's like my brain gave me a gift. (Here's my favorite experience if you're interested)

Also why do people open their eyes instead of keep trying to sleep?

For me, it's poor mental health and a terrible sleep schedule lol. I can't do it as often anymore because I have a job now, but I used to resist sleeping for some mental health reasons. So after I was too tired to want to be on my phone or anything, I'd end up staring at my ceiling thinking about whatever I was stressed out about at the time. I think that prolonged daydream-like state is what does it.