I think it's the wording of the rule. Too many IF, THEN statements.
IF the character has Darkvision in the dark, THEN they see as if in Dim Light.
IF it's Dim Light, THEN the target is Lightly Obscured.
IF the target is Lightly Obscured, the character is at Disadvantage on Vision Perception tests.
Now, if the rule just said: C"haracters with Darkvision can see in the Dark as if in dim light, which gives disadvantage to Wisdom (perception) checks to Sight", people wouldn't forget it so much.
But I think most players are like me. They're lazy readers.

They see: "Many creatures in fantasy gaming worlds, especially those that dwell underground, have darkvision. Within a specified range, a creature with darkvision can see in darkness as if the darkness were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned. However, the creature can't discern color in darkness, only shades of gray." and stop reading there. They (and I) don't go deeper by looking up Dim Light, etc. because it's not obvious that they should do so.
As far as the proliferation of Darkvision, I've scaled it waaaay back and replaced it with low light vision. The only PC races that have darkvision are Dwarves and Tieflings. All the others, except humans, have Low Light Vision. Humans just have regular sight. But that's a house rule and doesn't have much to do with the thrust of my OP.