There are several issues with this theory:
- Trump flew stuff to Mar-a-Lago from the White House. The materials aren’t just stuff that was already there, that he was too lazy to return. (That said, he could have just sent everything in his office in the White House, without any particular review of what it was. But → )
- He did, ultimately, return stuff. While we might accept a form of laziness before that point, there’s no way to end up with 10+ boxes worth of stuff still sitting around in a room and think, somehow, that you’ve returned everything. It’s like you borrow 20 lions from a guy, give 10 back, and try to profess that you just forgot that there were still some more hanging out. They’re not dormice, you’re going to notice when you’ve got 10 lions roaming about the house - even when it’s a reasonably large house. 12 boxes of stuff is like the size of a couple of couches, you’re not going to fail to notice them.
- Reporting notes that the documents brought back to the FBI were scattered around the house and in Trump’s bedroom. When executing a search warrant, the officers are held to a reasonableness standard. If they open a box, rifle through the papers, find one Top Secret heading, then they can take the whole box to check more thoroughly later, because it’s reasonable to think that someone would store similar documents in the same box. If you see a single paper on a nightstand and it doesn’t have a classified mark on it and the date on it is 2022, then you leave it where it is because there’s no reasonable argument that it’s relevant to the warrant. So if they didn’t do that - if they did take something from Trump’s nightstand - then that document was clearly pertinent to the warrant. You don’t have as-clear-as-day government documents scattered around the house willy-nilly, up to and including your bedroom, and think that you’ve returned everything.
You could try to say something like that he simply couldn’t be bothered to collect and sort stuff but #2 above goes against that. He did return some stuff. He did have collected documents that were stored in a particular room that could be padlocked and, thereby, protected. There’s some sort of logic associated with those documents ending up staying away from the government and staying together, in a pile, in a room.

The primary one you hear from the MAGA supporters is:
The documents were related to the Russia hoax/Durham investigation and Trump kept them because the FBI is covering Obama/Hillary’s tracks and by keeping them he could hold them all accountable and lock them up once he’s POTUS again.
Kash Patel is quoted saying (to paraphrase), “He declassified everything about the Durham investigation, so I went to the National Archives to get access to them.”
Ergo, Trump didn’t have what he needed available. They’re at the National Archives.