"Had plans" or "Have Plans"

[ChrisZhao]

In the following conversations, why the past simple tense is used in the sentense "I didn't have any big plans".

I feel present simple would be more nature.

Could anyone help, please?

Conversation 1:
A: Hey, Robert, what are you doing this weekend?
B: I didn't have any big plans.
A: We are putting together a birthday party for Mary.
B: That sounds like fun. Where will it be?
A: We thought it would be fun to have a pool party at Jay's house.
B: Oh good! Can I bring anything?
A: We will be providing hot dogs, hamburgers, and cake, but people can bring side dishes.
B: I am assuming that the dress is casual.
A: Dress casually and bring your bathing suit!
B: Wonderful. Just e-mail me the time and date and I'll be there.

[Alan]

Yes, I agree with you!