I think relative secularisation has something to do with it. People need something to follow and be a part of. Sociologically it is a religion, a set of beliefs and values to live one's life by. And humans will and do get obsessed and form their identity around those things.
Not everyone of course.
My ex was due to move to my town with me at the time we broke up over football, we'd looked at houses and discussed things thoroughly, but she bailed as she didn't want to leave her football team.
She would come to see me some weekends, but her team played on Sunday and she would be up with the lark and gone, having only seen me for one full day and with the prospect of not seeing me again for another fortnight.
Holidays, days out, appointments, trips away had to be planned around football.
Anything going on with me or 'us' was totally forgotten about. She wouldn't engage in conversations about anything else, largely. I once went to visit her, her having told me she'd be gone a couple of hours in the afternoon to play. I was fine with that. It turned out that it would be closer to 5 hours, and I found out about this via someone else, she'd not even thought about it, or what it would mean for me (I had nowhere to be, would just be wandering around town waiting). She was never physically violent, and strangely 'her' teams (local and national) winning or losing didn't seem to have much bearing on her mood (perhaps this is more of a male thing?) she was just obsessed with watching, playing, talking about and anything to do with football and horrid to me as a result of this 'one track mind'.
Last time I went to watch her team play she was awful to me all day, this was a common theme, she seemed to be embarrassed about me being there but if I didn't want to go, she would be annoyed. And they were beaten 20-1.
I have said since that I won't date anyone who's interested in football ever again!