People say they are not included so they can sell them to you for an additional cost.
Not always - sometimes it's because they love them and plan to use them in their new home!
In our case it was a light fitting. We'd bought a set of lighting that included ceiling light, two standard lamps and two table lamps. We loved them even though they were just cheapies from BHS, but they were carefully chosen to reflect our taste in decor. When we were selling we knew we'd be taking them all with us so we were very explicit that the living room light fitting was excluded but we would replace it with a standard fitting.
When we accepted an offer there was no mention of the light until just before the sale was concluded, and the buyer suddenly said he wanted the fitting, or the sale was off. We checked BHS but the set had long been discontinued so we couldn't have got another, but the buyer refused the explanation that it was part of a set, etc. etc. He was adamant we had to leave it or lose the sale. I suspect he didn't believe us and thought it was an original (Victorian flat, with ceiling rose) that we were trying to make off with.
DH and I argued all weekend - he wanted just to leave it and I didn't - then I had the idea of buying another similar light fitting and putting it in place of our one. He'd only seen the original once, and probably didn't remember it in exact detail, just that it looked "old". So I found one made of plastic in B&Q that looked close enough and only cost about £15, so that's what he got when he moved in. He never said a word about it, but I'd love to have seen his face when he finally found out...