~George S. Patton
A lot of times, it’s guessing. If something isn’t in modern language, people attribute it to Shakespeare, which is why this,
which is Walter Scott, is attributed to Shakespeare all the time.
It gives the glurge more credibility.
A recent instance of a frequent misattribution occurred when Emma Watson spoke before the UN about feminism. She said, " Statesman Edmund Burke said: ‘All that is needed for the forces of evil to triumph is for good men and women to do nothing.’ "
Burke made a statement that means roughly the same thing. But there is no record of him saying exactly or nearly that. It’s much closer to a statement by John Stuart Mill. The Only Thing Necessary for the Triumph of Evil is that Good Men Do Nothing – Quote Investigator®
You tell me why she didn’t verify the quote.
It may have been better to say, "It is said: ‘All that is needed for the forces of evil to triumph is for good men and women to do nothing.’ "
None of the historical quotes include “women” to start with and trying to succinctly change the attribution that everyone “knows” to be correct would cause that to become more important than it deserves to be. A bit of a quandary.
And that’s why this very board has such a hard-and-fast rule against tweaking the wording in quote boxes.
Long before the Internet, that statement was attributed (on motivational posters) to Robert Kennedy, and in newspaper stories to John F. Kennedy. JFK himself credited it to Burke; RFK used it in a speech, I don’t know if he credited Burke or just moved on.
I remember a line like, “No matter who first said it, Dorothy Parker always got the credit.” Unfortunately, I can’t find a cite anywhere, so I’ll attribute it to Ogden Nash. That’s only fair. Dorothy Parker is often credited for, “Candy is dandy but liquor is quicker,” when that was actually written by Nash.
See how it works?
However, Dorothy Parker did say “What fresh hell is this?” when the phone rang, and the writers of TBBT put it in the mouth of Sheldon Cooper. Now you see it all the time on T-shirts, variously attributed to “Sheldon Cooper,” and Chuck Lorre.
There is a phenomenon known as Churchillian Drift in which a wise or witty saying is attributed to a more famous person than the originator .
It was a problem even in Dorothy Parker’s time:
Funny, it used to be known as Disraelian Drift.
We’re had various threads on the SDMB about the supposed Dorothy Parker quotation “This is not a novel to be tossed aside lightly. It should be thrown with great force.” We can’t find any proof that she wrote it. Here are some of those threads:
Why? Because it’s like Shakespeare once said, “Bitches be crazy.”
It isn’t obvious to people who have no prior experience reading texts written before the late 20th century, and / or haven’t read enough to develop instincts for language and writing style in general. (I’m reminded here of one of my students a few years back, who plagiarized from a nineteenth-century book about Shakespeare’s heroines that she found on Google Books, as if I wouldn’t notice that both the prose style and the attitudes about gender roles were as Victorian as they could be. Some people are basically tone-deaf to differences in style and register.)
What was the quote?
Yes.
Glurge made by me and attributted to me by me has little staying power in my experience 
“Nothing is easy” by Billfish678. Yep that sucks.
“Nothing is easy” by Albert Einstein. Man what a powerful insight! And it’s SO true. And if anybody would know that it would be old Albert!
See what I mean?
Jethro Tull STOLE that from Albert Einstein?
That’s it. I’m throwing away my copy of Aqualung. I just can’t trust that Tull guy any more…
We must drift with the times
This particular coinage was by Nigel Rees
I thought it might be something like that, but had also hoped by now people would have enough of an idea that famous people really didn’t say all the things they’re alleged do. Silly me…
Are we sure that some people aren’t introducing these misattributions as a way to track how much “their” idea has spread?
If I post on Facebook a correct quote from Jefferson, and then Google that, I’ll see it all over the place where other people have also used it. It’s hard to say whether my use of it went anywhere.
But if I post “Just believe in yourself because you’re awesome” attributed to Thomas Jefferson, I now have a unique search phrase and a way to see how far my post went as other people repeat it. The million other boobs who like, reply, repost, plagiarize, retweet, etc. are not going to bother fact checking me and will simply be making an honest mistake. If the quote has legs, it eventually takes on a life of its own, and I’m sure I will feel quite special in seeing three million hits on Google for my little meme.
This kind of thing was certainly a motivation for chain e-mails, and I haven’t seen a “forward this or you’ll have bad luck” e-mail in a very long time.