She is probably staying for one of many reasons or a combination of them.
A) She wanted more kids and wanted them to all have the same dad.
B) She didn’t want to let Lauren “win” by admitting defeat and leaving Kyle.
C) She didn’t want to risk Kyle getting with Lauren in a proper relationship if she ended the marriage and also risking Lauren being able to play stepmum to her kids.
D) She doesn’t actually love him anymore but is happy in her lifestyle with the money and security.