DH and I are two separate individuals with our own goals/personalities/whatever, but we still view it as OUR money. We literally had a conversation about this earlier this morning (because my MIL is annoying me with comments about how husbands have to be the breadwinners).
But your accomplishments are not independent. Say my girlfriend volunteers to do a bunch of overtime to earn a lot more money. She would be less willing to do that if half of that money went to me, for doing nothing.
Suppose I have the chance to take on extra responsibilities at my job, with extra pay. I would be less willing to make that sacrifice, if I only get half of that extra money.
Pooling money would only make us weaker (financially).
Also, spending money required to participate in the hobby is kind of indistinguishable from the hobby itself in my book. If you're annoyed at the spending, it means you're not fully supportive of the hobby. If you were, the cost wouldn't bother you because it'd be a previously agreed upon mutual decision.
Your argument makes no sense. Whatever hobbies my girlfriend wants to do is fine. It's paying for something that has nothing to do with you that bothers us.
Suppose we had the same hobby, we both take dance lessons. Neither of us would be "annoyed", obviously. But I want to take $100 of lessons a month, she wants $250.
My paying for her (or vice versa) is what we don't like.
What happens in this split money scenario when someone gets laid off? Or an emergency happens? Do you each maintain a separate emergency fund of x*(your half of monthly expenses) and then once that runs out, what happens?
Not a problem, we're both on the ER track and mustachian so we save over 50% of take-home and could live with zero income (even at current spending levels) for several years.
If one of us somehow ran out of money, which would be almost impossible, then the other would temporarily pay for required expenses only (rent, groceries, etc.). But it would be expected for the other person to get a job ASAP.
It's just a question of outlook. Neither of us feel that being in a relationship is equivalent to paying for each other's expenses, which is what many people seem to think. And that attitude makes us a stronger economic unit.