Why do women over the age of 30 start wanting to know how much money you make as a man?

Because at that age women are getting pretty desperate and are looking to have children whilst they still can. A man with money is a man that is seen to be able to provide the best nest for the chicks. Though of course not necessarily the best relationship for the woman.

Because in your 20s it’s kind of expected that most people are starting out, in lower level positions, have fewer resources, etc. But as you get older, you should be making progress and moving forward in your career path. Women don’t want men as partners who won’t pull their financial weight, so if you still live in your mom’s basement, work at a low level job, drive a 20 year old car, that tells a woman a lot about your drive and motivation. 

The question about what kind of work you do is a general getting to know you type of question, and it’s not the same as asking how much money you make. And yes, if you’re still single in your 40s, you shouldn’t be surprised that women want to figure out why that might be.

Because the look of women are declining when ageing and for men looks are more important than for women. Hence, women are less likely to attract men with their declining physical beauty. I am not saying that personality is not important for women, but evolution and biology programmed humans (men and women alike). When women's looks are declining, they are less attractive and their options to attract men is much more limited.

And therefore the possibility to attract a man who funds or helps them fund a decent life style is limited as well, and especially women need financial security because they earn on average less money than men and also have less pension. That is why women are increasingly interested or even worried about their financial future. They need to know whether they can build up a family, raise kids with that man and whether this man can help them to have a secure retired life later on.

Asking what someone does for a living is an extremely common question when getting to know someone. You, by not asking, are in the minority. And asking what you do for a living, is NOT the same as how much money do you make.

You have been single over 3 decades. I don't think the fault lies with these women

One's profession at that age says a lot about a person.  If you say you are a cashier at In N Out when you're 34, that say s a lot about your smarts and ambition.  If you say you're in construction or an accountant, that says something entirely different about you. Money may be part of it for some women, but really it's just a conversation starter for most.  It's a common question for people who are trying to get to know someone else on any level.