TOEIC question: Why don't we go out this evening vs. Why aren't we going out

Hi, here is yet another question:

Why don’t we go out this evening?

Why do I have to use the present simple here?

Couldn’t I use the present progressive here too?

So, my question would read:

Why aren’t we going out this evening?

Would that be correct and if so what is the difference between both constructions?

Thanks a ton,

“Why don’t we go out this evening?” is a polite suggestion. So far, there’s been no plan to go out. You’re introducing the idea.

If you say, “Why aren’t we going out this evening?” it means that there has been some plan (concrete or assumed) that you would be going out this evening. Now you have found out that for some reason you are not going out, and you’re asking why.

Remember that when the present continuous is used for the future, it often indicates that activities have already been planned or arranged.