My immediate thought in answer to this question is, It's a look that gives me physical euphoria. It's what I am 'programmed' to be attracted to. Looking back, the ten year old me was enthralled with my very boyish classmate and that was the start of it!
It does go deeper than this though.
Butch women are usually a certain type of person. There's obviously vast differences between them, but despite this, there's a certain thing about the majority, and that personality and outlook on the world and on life, is something I am attracted to. They make me feel 'safe' and they usually are more traditionally 'masculine' (note I do not mean 'like a man!') in many ways which balances me and my outlook and my personality.
Of course 'butch' generated from women hiding dating women, by one of the women presenting as male, but the majority of women I've dated have been of this look since they were very young or if they weren't, they were as soon as they felt safely able. It's who they are, they're not into many traditionally feminine clothes, activities, jobs or any mentionable thing humans do. It's naturally who they are, and that mentality and self-awareness is also something I admire.
I'm in my forties, have been out since my early twenties and I've never dated anyone who presents as feminine, 'girly' or any other description. I doubt I ever would. I guess the same as some women may say they prefer a certain type of man, I just prefer a certain type of woman.