Ok….I do this.
It’s an instinctive immediate reflex - if I’m in a group of people talking about loving a particular restaurant that I haven’t actually been to and don’t like the look of, I’ll usually go along with it, talk as though I’ve been there as well (by repeating things I’ve seen online about it or heard from somebody else).
It comes out of my mouth with no conscious thought and afterwards I’ll think “oh fuck why did I say that, what a pointless stupid lie!”, I don’t ever plan to do it in advance, it just happens in the normal flow of the conversation.
In my case I think it comes from having a really weird childhood (not unhappy or troubled, but very unusual in the quite small insular town we lived in) - I felt really intense embarrassment about how odd and different our family was, never invited people over, never told the truth about how we spent our weekends, and just generally lied to fit in.
People definitely still knew about some of the weirdness and I was bullied a bit, but I mostly hid it and just lied to seem more normal.
Its something I’m consciously trying to stop, and I don’t lie about big things or plan in advance to lie - it’s a habit borne of a form of people pleasing/fitting in so just going along with stuff.
Obviously you may not want to be with somebody who does that, fair enough, but just offering a possible explanation.