Why don't people with bad memories adopt the phrase and concept 'I don't know' or 'I'm not sure, I can't quite remember' or 'well my memory's shocking but I'd imagined...' ?
Why would a person who knows their memory is bad, pretend to themselves that they know for certain what happened at some point in the past? Knowing and having a poor memory are opposing concepts.
Why wouldn't you, why don't you, adopt 'I don't know' and 'I'm not sure' to fill the gap between those concepts?
Why would you, why do you, adopt 'make up any old rubbish / insert other event / insert what you imagine might have happened / insert what you wish had happened, to fill a space, then adopt / believe this insertion yourself'? (How can you not be aware that you're making it up, or deliberately inserting it? Why would you choose to do that?).
Are you just nor very interested in truth? More in storytelling? Wish-fulfillment?
I really am curious, as I know a few people who do this habitually (all the time). That is, say, with shocked look upon face 'but I thought....' when what they mean is 'I really have no idea but had imagined / speculated / surmised that one possible explanation was...'
Basically, instead of acknowledging 'I don't know', or 'I'm assuming but I know I'm assuming, so could be wrong', they make shit up to fill gaps in their knowledge, then believe their own shit.
Why are these people's brains unable to accommodate uncertainty? Why are they not aware that they are merely hypothesising, to fill that uncertainty gap?
That's a different thing from deliberately lying to other people.