We are. In standard worlds monsters are rare and not encountered by the populace at anywhere close to the same rates as PCs.
In standard D&D worlds libraries are common, you can buy guides to monsters for 50gp, every large town has clerics, wizards and other specialists who can tell you about monsters and there are guilds and organizations specifically to help you do your job well. It beggars belief that intelligent professional adventures can only remember 2-4 things about the things they deal with as part of their job.
It feels like you think this is the way the world should work:
"Oh look -- a skeleton"
"Wow. What is that, I've never seen it before."
"Actually we fought them a year ago"
"I must have forgotten. I never bother remembering things"
"Also, everyone tells stories about them. They are in so many stories!"
"I never listen to other people"
"They are in VOLO'S GUIDE TO MONSTERS"
"Never read it"
"We're part of a guild that regular hires out to fight them. We've met dozens of people who've fought them"
"I never talk to other professionals or share info with them"
"Clerics know how to summon them"
"I never talk to clerics"
"Most large towns have libraries you can visit"
"I never do so"
"Do you know anything about ANY MONSTERS?"
"Of course! Over the last five years of training to be a paladin, I have learned a little about Orcs AND Goblins."
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Unless your world's people are significantly stupider than people in our world are, the extreme lack of knowledge about monsters that most D&D games expect is really unrealistic and is purely a gamist way to have fun. I play the rules as written, but I don't try to pretend then are anything other than rules and are nothing like realistic!