Possibly because Cersei wants to make Dany look like a villain by forcing her to kill innocent civilians and burn down King's Landing. This was a point of interest presented through various dialogue in three scenes. (Emphasis Mine)
8.04 Transcript - Group Meeting - Tyrion pleads with Dany
Missandei: When the people find out what we have done for them, how we
Daenerys Targaryen: Cersei will make sure they don't believe it. We
will hit her hard. We will rip her out, root and stem.
Tyrion Lannister: The objective here is to remove Cersei, without
destroying King's Landing.
Varys: Thankfully, she's losing allies by the day. Yara Greyjoy has
retaken the Iron Islands in her Queen's name. The new Prince of Dorne
pledges his support-
Daenerys Targaryen: No matter how many Lords turn against her, as long
as she sits on the Iron Throne, she can call herself Queen of the
Seven Kingdoms. We need the capital.
Tyrion Lannister: I watched the people of King's Landing rebel against
their King when they were hungry, and that was before winter began.
Give them the opportunity, and they will cast Cersei aside.
Jon Snow: We'll surround the city. If the Iron Fleet tries to ferry in
more food, the dragons will destroy them. If the Lannisters and the
Golden Company attack, we'll defeat them in the field.
Tyrion Lannister: Once the people see that Cersei is our only enemy,
her reign is over.
8.04 Transcript - Cersei, Qyburn, and Euron - Cersei reveals her intent
Cersei: Our message was well received then?
Qyburn: Your people would of heard the usurper is coming. They are
grateful for your protection within the walls of the red keep.
Cersei: You're certain about the dragon?
Euron: I saw it sink beneath the waves.
Cersei: that must of been glorious.
Euron: The glory is yours, my Queen.
Cersei: When the war is one the Lion shall rule the land, the Kraken,
the sea, and one day our child shall rule them all.
Euron: She's coming for you.
Cersie: Of course she is. Keep the gates open. If she wants to take
the castle she'll have to murder thousands of innocent people first.
8.04 Transcript - Tyrion trying to persuade Qyburn to persuade Cersei not to force Dany's hand
Qyburn: My Lord.
Tyrion: Queen Daenerys demands Cersei's unconditional surrender and the
immediate relase of Messandie of Narth.
Qyburn: Queen Cersei demands Deanerys unconditional surrender. If she
refuses Messandie of Narth will die here and now.
Tyrion: Qyburn, you're a rational man.
Qyburn: Or so I flatter myself, my Lord.
Tyrion: We have a chance here, perhaps our last chance to avoid
Tyrion: Help me. I don't want to see this city burn. i don't want to
hear the screams of children burning alive.
Qyburn: No, it is not a pleasant sound.
Tyrion: I, I don't want to hear it. Help me save this city.
Qyburn: My Lord, I am only a mouth-piece for our Queen.