It happens a lot less at higher levels, but back when 0 was death and Wizards had 1d4 hitpoints, most sword hits killed them in one blow. Wizards often had AC 10 and it was really easy for monsters to hit them. So choosing to attack them meant you were choosing to kill them.
Well, at really low level, sure. Some players learned to keep their wizards around corners and such in many situations though.
We also coveted items like Cloak and Boots of Elvenkind (not usually targeted unless found), Rings of Regeneration (couldn't rely on the Cleric to cure you, they have few spells too), the Invisibility spell, etc.
Obviously some people played with optional rules about not dying at 0 that helped to mitigate that, and it became the standard rule later. But when I started, the DM choosing to make an attack roll against the Wizard meant you were purposefully choosing to kill off that wizard.
Yes, I'm sure that we used that rule too. I don't remember it, but we probably did.
We had a couple of DMs who were bastards who would have enemies purposefully kill off Wizards since "They are wizards, wouldn't you target the guy with the fireballs first?"
That sucks. I'd spill my coke on that dude and his stuff. Repeatedly.

That's bullxxxx.
Not to say that the Wizard should be protected by the DM, but that's just asking for it.
Since people were so sensitive about it, it became a thing that unless the enemies had no other choice, they would attack the non-wizards first.
Different experiences.
It happens, though fairly rarely in the games I've been part of. Most combats involve the PCs coming across enemies rather than enemies hunting them, so the enemies rarely have the time or opportunity to surround them.
Though, when these incidents happen they are considered to be some of the more deadly encounters...specifically because the DM will often say "well...these ones at the back see the Wizard first, so they attack the nearest target". Which often involves the Wizard dying...or at least dropping.
That's not my experience in 4E (earlier editions, sure, a surrounded wizard was toast unless he could go invisible or something). The 4E first level Iron Mage was one of the first ideas talked about.
As for not being surrounded, that's a result of the segregated room concept that many modules have. DMs are trained to run and design adventures that are so linear (i.e. the kobolds in room 3 do not attack because they do not hear anything in room 2, err, what?).
Now that I think of it, I did lose a high level Wizard in 3.5. The DM had an idea that certain spells could get past an anti-magic shell, so before my PC's init even came up, he had his NPCs blast the stuffing out of him.
I also had a 9th level or so dual class wizard cleric mystic theurge buy the farm as well in Pathfinder. He had saved the party's bacon on a lot of occasions (due to how many spells he had, he could afford to burn them when necessary), so the DM pulled out all of the stops in Strahd's castle and again, it was two powerful back to back damaging spells before my PC could react to the first one.