There is no absolute rule/line that says they do get it; there are stories and sidenotes that make it clear that this is done, though.

First, the President has Top Secret clearance, that is not revoked automatically upon their departure from the office. The incoming President can revoke their Top Secret status, but nobody has. It's useful to have past Presidents close by for advice and consultation on issues they are particularly well-versed in, so maintaining their Top Secret clearance is normal.

Second, anyone with Top Secret clearance can request a debrief from the CIA, whenever they want. The CIA produces the PDB, and the same individuals who produce and present the PDB content are the ones that would debrief the Top Secret clearance holder who made the request.

Third, people like George HW Bush was said to get it every day, and "read it religiously". Now he was also head of the CIA prior to becoming President, so this was His Thang. So naturally he kept his mind busy with the thing he was passionate about.

Fourth, is has been noted that other Presidents - like Bush 43 - never asked for it, after leaving office. This mention alone indicates there is some expectation that it is normal for a past President to request briefings.

Fifth, there was some uproar over Obama getting debriefs after he left office that let a bit of the unspoken gentlemen's agreement between past Presidents out into the light. Trump lied about it to cover the fact Obama still has access to this data/analysis, but it doesn't change the fact that Obama still had access to this information after he left office. (I expect Trump lied about this to cover his own ass, and nefarious intents for what he plans to do with the information once he is out of office.)

So, in short, my title might have been more precisely accurate if I'd gone with something like: "Past Presidents retain their Top Secret clearance after leaving office, unless it is revoked by the incoming President, and as an individual with Top Secret clearance they have the right to request either the PDB or a debrief by the CIA officers who create the PDB, however this right has not always been exercised by past Presidents."

(This CIA analysis of the PDB is a pretty good source for how it is created and given to the President. But it does not go into how the document is shared with anyone BUT the President.)