Yeah but your personal experience doesn't really reflect the amount of people who get attacked by third party combatants while standing.
I've seen A TON of people standing get sucker punched. Trust me, I haven't seen any shortage of it. Two guys standing and fighting each other, someone else gets into the mix quite often. Completely unverifiable number here - about half the time. How often has someone kicked a guy in the guard, hit a guy in the mount in the back of the head with a bottle, etc.? About 99% of the time; again completely unverifiable.
I saw a lot of stuff bartending in college bars on weekend nights for the better part of 10 years, and being a guy in the bar for quite a few years too.
What I saw over the years could be a completely isolated thing, or it could be indicative of everywhere. No idea. I just know what I saw. Over and over again. Same things happened at concerts, festivals, etc. Maybe times have changed, as I haven't frequented bars in the last 10+ years, but I doubt everything has changed that much.
From what I've seen and where I've been, if an intruder came into my house, attacked me in a parking lot with no one else around, etc., I'd take him down or throw him using stuff I learned wrestling, and mount and fire punches like there's no tomorrow until the threat was completely neutralized. No doubt about it.
Whenever I've had no choice but fight inside or outside the bar or other public places, I've made sure I stayed on my feet and could easily face anyone else coming at me. Have I been blindsided by someone else? Absolutely. Was I able to get out of the way to avoid follow ups? Absolutely. I wouldn't have that luxury on the ground. About 10 years of wrestling and 6 more of coaching on the mat gives me enough experience with grappling to know I can't get easily away from repeated kicks to my head while I'm grappling with someone. I'm pretty much stuck. And I think there's a psychological thing that tells people to jump in when the fight goes to the ground far more than when 2 people are standing. But that's another thread.
For me, the best place to be in a fight is on my feet with my opponent on the ground, conscious or unconscious.
All my personal experience and opinions. I'm sure others have contradicting experience. Go with what you've seen, not what you've heard. I'm just one of the countless guys on countless forums.