NOTE: SPOILERS follow.
Cersei could easily have killed Daenerys, Drogon, Tyrion, and Grey Worm during the final parley, yet didn't do it. If she had, she could probably have decisively won the war right then and there (Jon Snow could not have beaten her with the remaining northern forces alone).
It's the single most egregious strategic error of the entire last two seasons of Game of Thrones, which is saying no little considering all the mistakes made by Tyrion and Jon Snow, among others. Is there any possible explanation for it? Let's try:
Cersei is too ethical to violate the laws of war in this way.
Response: Don't make me laugh.Cersei was too stupid to think of it.
Response: Possible, though unlikely. She's not THAT dumb. Even if she was, one would think that Qyburn or Euron or another adviser would point out this obvious solution to her problems.She loves Tyrion and wants to spare him.
Response: Unlikely given her longstanding hostility to him, and the fact that she just hired Bronn the mercenary to have Tyrion killed. But even if she does want to spare him, why not open fire on the others once Tyrion walks up to the gate and there is distance between him and the rest of his party?
The real explanation: The showrunners didn't think of this and/or they wanted to put the two queens close to each other for dramatic purposes. Having Daenerys send an expendable courier to convey her demands to Cersei would not pack the same punch, even though that's what would realistically happen.
While we're on the subject, Dany's and Tyrion's decision to approach this close was ALSO stupid. They couldn't know Cersei would show such unaccustomed restraint! But tactical idiocy has become typical of Tyrion over the last 2 years.