She is the princess because the title/claim is backed up by others. Stormwind was ready to march on Lordaeron for the sole purpose of giving Calia back her land. Even when Lillian, who was an esteemed member of the Forsaken saw Calia, she bowed and called her my lady.
The same people who call her Princess? if you read Before the Storm, Forsaken call her Princess. In Shadow’s Rising she’s referred to as Princess of Lordaeron and Councilor to the Horde as a representative of the Forsaken.
Some members of the player base may not think she’s the princess, but the Forsaken NPCs, aside from a few like Belmont, don’t like her. But why is that? Because Belmont followed a different leader and doesn’t want his work for the past 20 odd years to be uprooted. He doesn’t want things to go back to how they were when the Forsaken were human. Why? Who knows? Maybe he wasn’t as important as he is now.
If you haven’t read Before the Storm, it’s worth the read. There is a fantastic bit, that really helps portray Calia’s character, just before she’s unfortunately killed.
“You are my people, and I want to help you,” Calia pleaded. “I only came to observe, to begin to get to know the Forsaken of Lordaeron.”
“Undercity,” Elsie said. “We live in the Undercity.”
“You didn’t once. You won’t have to live in the shadows anymore. Just—please. Come walk with me. Parqual, the Felstones, all the others—see them? They’re defecting. Anduin will shelter and protect you all; I know he will!”
“But—the Dark Lady—”
As if in response, the horn sounded three sharp blasts. Elsie turned her gray-green face back toward the wall and the Forsaken banner that had been unfurled.
“I’m sorry, Your Majesty,” Elsie said. “I can’t betray my queen. Not even for you.” She turned and shouted, “Retreat! Retreat!”
Elsie was frightened of Sylvanas, which makes sense, in the book previously she (Sylvanas) took away all memorabilia and mementoes of Lordaeron because she feared that the Forsaken would defect. Heck, even the Desolate Council was a bad thing because she feared they’d stray too far from her.
Sylvanas cares about the Forsaken because she sees them as her people. Calia cares about the Forsaken because they were her people when they were alive. However… Calia didn’t murder Forsaken who wanted to leave. Sylvanas murdered Forsaken who wished to go live with their human family members.
She lost control of the Forsaken and she ended them. She views herself as God, which is a terrible trait in a leader (great trait in a villain).
This is why they put her on the Council. It’s an internship. The end goal of the Forsaken was always to become normal again. However that shifted and they went into this “Lol blight everything, we’re evil zombies” thing. Now Blizzard is changing their narrative to actually have a story instead of being the villains.