We already know that Jon delivers a wight to Cersei in King's Landing thanks to the season seven finale preview, but the fact that Jon, Dany, Tyrion, and everyone else are operating under the assumption that she'll care about the White Walker invasion after seeing the wight is baffling. Cersei has already proved, time and time again, that she's power hungry to the point of insanity. Do they really think she'll put aside her ruthless tendencies (which she hasn't let go of since the show began) after seeing a single wight?
There's no way of knowing for sure until the finale airs, of course, but this is Cersei Lannister — she's the one who blows up the Great Sept of Baelor in season six, remember? Not to mention, she is already well on her way to becoming the Mad Queen. If she finds out that the White Walkers are an immediate threat to the North, and will likely take out Jon, Sansa, and Dany's forces long before her own, there's no way she'll simply agree to help them. She looks out for numero uno at all times and will likely celebrate the idea of the Night King decimating her opposition.
No one is even bothering to anticipate that? They'd rather just march right into the belly of the beast? OK.
