Why not "just a wishful thinking"?

I don't know, but maybe 'wishful thinking' is like bread and water. You're not likely to say you had a bread and a water for lunch.

Thanks.
I think JTT might have a ready answer.

Roger's got it, Ori - a mass noun.

Don't you trust Roger?

Almost certainly not a bread, but many liquid drinks in speech are shortened, their counter dropped - a beer, a whisky, a water, a scotch and water, a bourbon straight up, a red wine, ... .

counters - a bottle of, a shot of, a glass of, ... .