Titles are important, Cersei
When you think about it, neither Jon Snow nor Daenerys Targaryen ever rebelled against Cersei Lannister.
She was proclaimed "Queen of the Andals and the First Men, Protector of the Seven Kingdoms." That is not the entire royal title and the entire royal title has a meaning.
The title all the Targaryens and Robert held was, King of the Andals, (the Rhoynar) and the First Men, Lord of the Seven Kingdoms and Protector of the Realm. All of these elements are important and are rooted in the history of Westeros.
The King/Queen of the Seven Kingdoms is the actual royal title. The will of the people elects a person to royalty. Their kingdoms rose and fought each other even before the Andals and de jure and by tradition, only by the will of the people does a person become a sovereign. The title that awards a person the singular ruler of all seven of the kingdoms has to be based on the will of the people, in name at least. And the people who make up these kingdoms are namely: The Andals, the Rhoynar and the First Men.
The Lord/Lady of the Seven Kingdoms is a title referring to a later construct: Feudal society and vassalage. First came the people and their leaders. One king, one village, one castle, one people. Then kingdoms grew, became larger polities, with a growingly intricate system of ranks and loyalties. In this system, a lord swears fealty to a more powerful lord. In short: The Lord/Lady of the Seven Kingdoms is the ultimate liege in vassalage, s/he is the one to which all the Lords/Ladies paramount and the Prince/Princess of Dorne swears fealty. It is the title that puts behind the throne the political power over the realm.
And then, Protector of the Realm. It is the title of the highest military leader. The commander-in-chief and aside from all the armies, all the Wardens, the Kingsguard and the Night's Watch are his/her subordinates. (Although the Night's Watch is a very special case.)
Now Jon Snow is declared King in the North. Arguably, he has a stronger claim than Cersei since his title, based on the will of the people was actually declared by the people. Arguably, but it doesn't matter. Because Cersei was never declared Lady of the Sevsn Kingdoms. De jure, in her rule, the North is a kingdom with a lord paramount, one that she never formally claimed to be a liege of. Even being generous with her, the worst she can claim Jon Snow is is a contender for the sovereignty of the North.
And so is Daenerys. Cersei can claim all she wants that Daenerys is a foreign invader, she cannot formally claim homage from the nobles of Westeros against Daenerys because Cersei was never proclaimed Lady of the Seven Kingdoms.
Whereas Daenerys? She receives homage from the Martells, the Tyrells, and the Starks voluntarily to whom she's then a liege. Which puts the Tarlys, the Tyrells' vassals in open rebellion and legally speaking, makes their execution justified. And Cersei, albeit she claims sovereignty over the people, de jure, a conspirator in a rebellion against a rightful liege lady of Westeros.
These things may not matter the moment the blood hits the floor again.
But in long dynastic struggles, in matters of perception and solidifying their rule, monarchs have to count on them. Even after winning the rebellion, Robert Baratheon had to find a legal justification for his actions. And even with that, he himself admitted that some continued to call him a usurper, seventeen years after he won.
So, as for Cersei... Something something, Dracarys.
