I don't understand why people think his lifestyle is generally incompatible with having a girlfriend. Of course it is incompatible with most people in the western world, but it seems some people think it is more so with women. Someone care to elaborate?

Society expect women to own more: beauty products, clothes, shoes... There is simply a lot more pressure on women to invest and diversify their appearance.

But there are women who live with far less too, even if more than what I depicted in this post.

and kids!

And decide to settle and buy a house...

Is a house actually necessary? I've been content with living in a rented flat so far. Or is that mainly an American thing where outside of larger cities space is abundant?

Depends on your situation and how long you plan on living in one place. In the US there are tax advantages to home ownership, my mortgage + property taxes are currently less than what I would pay for rent for a comparable house. Situations differ by individual and region. Some places like San Francisco are seeing rents go through the roof, while some places like Berlin are still very reasonable.

Simplistically? Rent goes into someone else's major savings account (house), mortgage payments go into your own.