No, as a player I can not freely ignore or change anything I don't understand.
Then this makes even less sense to me.
If I am collecting gossip and the DM gives me rumors about Sammaster returning and I don't connect it to the Cult of the Dragon, I can't tell him his plot isn't happening.
Based on a single check and the name Sammaster, my character would have also failed to connect the name to the Cult of the Dragon. Sounds like either a bad adventure or a DM who has failed to make it clear why something is important. Happens all the time. DMs learn from the experience and the game moves on. It has nothing to do with the setting.
I've had players come to me and say "I don't know much about the setting we're playing in. Will I still be able to play? Should I read a whole bunch of books?" My answer has always been the same "Prior knowledge isn't necessary. Here's a players guide I've written up (never more than 20 pages) to give you the information you need to make a character appropriate for the campaign. If you REALLY want to, you can read book X, but it's really not necessary." I've never had a player come to me after a few game sessions and say "I can't enjoy this campaign because I don't know enough about it." I've GM'd in Forgotten Realms, Eberron, Dark Sun and Golarion. Each setting has a hefty library of books that can be intimidating to a new player. I've never had a problem and I've GM'd for brand new players to tabletop roleplaying games.
So if you're struggling to keep up in a given game, I'd disagree that the problem is inherent in the Forgotten Realms. Any setting that is actively supported by more than a token number of products will have the same issue given enough years. A DM can easily manage this without issue.
WHY run a specific setting if not for the shared background with the players? "Hey, I'm going to run star wars". "Cool, and I play a Jedi?" "Well, nah no jedi, sith, smugglers, droids or the force." At that point, why put it in the Star Wars Universe? Same here. If you aren't going to cater to the FR parts, why play FR. If you're going to use FR as a shorthand so you can say "Cormyr" and your players immediately know what you are talking about, they'll have expectations such as the Purple Dragon Knights being around.
You said your players knew highly detailed information about a cabal of semi-secretive wizards and then looked for very specific information about things their characters had never seen in game. You then equated me saying Jedi, Sith and Smugglers don't exist in Star Wars? The two are not equatable.
But let's use a different example. Let's say it was elves, dwarves, Cormyr and the Weave. When I run a campaign I read up on a setting. Whether it's Eberron, Golarion or the Forgotten Realms is irrelevant. I'll read a supplement on the region, and then run the game. Things not covered in that supplement aren't relevant to me. What I've read is what informs on my campaign. I then tell the players what the premise of the campaign is and ask them to create characters that have a reason to be there and participate in the setting. In this case, if my campaign was in Aglarond, the details of Undermountain are both irrelevant to me and my group. Cormyr would be of some importance (as a major player in the Inner Sea region), but the detailed inner workings of Cormyr certainly won't be while dwarves will be of little to no importance but certain elven subraces will be extremely important. The Weave will be a thing that exists but won't take center stage of the campaign unless I cause it to. But I am taking full advantage of the information on Aglarond itself.
4e is one edition ago. Try three settings worth of it being the earliest out as I mentioned, and that's not even all of it. It was out for AD&D, AD&D 2ed, 3.x, 4e and 5e. So it's been around for 5 editions, and in at least three it's the first out so that people wanting to play the new system and are looking for more, it's the first setting expansion.
https://en.wikipedia.org/wiki/Forgotten_Realms
Oh. I see. So you're saying because of 4 edition's worth of support FR is more popular than any other campaign setting. I'll concede that point but it's also irrelevant. When gauging which setting to support in 5e, WotC will look at what has been most popular in the past and then make that the primal focus. So yes, it is a self-reinforcing situation. But after 20+ years it's also irrelevant that it's a self-reinforcing situation because that's where the money is.