I think it's very realistic for someone who's never had to worry about money to think that it doesn't matter, I've known several people like that. She's always come across as very sheltered to me, and even when she and Jack had financial problems (when they found out they were having triplets, when they wanted to send Randall to private school) it never seemed to occur to her to get a job or otherwise bring in money. It was always Jack who came through, usually at a high personal cost (asking his abusive father for money, giving up his dream of starting a company), and she didn't bat an eye. It was only after Jack's death, that she finally had to step up.
Maybe, when she tried to get a marriage license to marry her fiancé, she found out that her and Kevin's divorce was never properly finalized? (I hope not, that would be too soapy and surely lead to more of Kevin's romantic escapades.)
The reason is that the show loves to mess with the audience.
I was about to say that she's done a lot for Kevin, but then I remembered that she was his paid assistant.