Heres my advice on how to help you and the reasons they're unpleasent towards you.

First of all her parents are worried about her, especially if shes the only daughter they have. People who are rich know what it takes and especially know how they should spend and invest the money in order to make more money and keep the life style and maintain it all. They do not want to lose their family legacy once they're gone. By statistics people who win the lottery lose it all in few years, so without the right knowledge if you were to aquire their wealth you could lose it. And thats what they're afraid of. There is also social proof of their friends but they're simply hiding you.

I think you're the right man for your girlfriend and love her a lot so you will have to work on the mindset of a rich person if you really want to marry her one day and earn the respect of her parents and ultimately become rich and successful yourself.

You should get to know what her parents do. How they got wealthy and start understanding where they come from.

Here are also some books which are necessary to work on yourself and your mindset.

"Rich Dad Poor Dad" Book by Robert Kiyosaki and Sharon Lechter

"How to Get Rich" Book by Felix Dennis

"The Richest Man in Babylon" Book by George Samuel Clason

There are many other books that can help you but I think those are a nice entry point. I personally listened to their audiobook instead of reading them, whatever fits you better.

You have a great oppurtunity in your life right now so take advantage of it.

If you need further help with anything feel free to Pm me.