Many reasons.
1: Dressing manly (not putting on makeup, short hair, jeans and shirt) is comfortable, low-maintenance and cheap. When your social circle accepts it, why not do it?
2: It's a subculture thing. Just like some subcultures shave their heads, wear specific band attire or head gear, "Butch" lesbians dress a certain way to signal that they are part of that subculture.
3: During a time when lesbians had to fear violence from homophobic people on a much more prevalent basis, some of them dressed and trained to be physically strong, imposing and able to defend themselves and others.
4: It's a way to dodge unwanted attention by men, who they are not interested in.
As a man, I can totally understand those reasons, especially number 1. I once tried having long hair and got too annoyed with it, and putting on make-up and dressing up cuts into my video gaming time.
For a specific lesbian looking like a man, it might be a combination of those and other reasons. If you really want to understand them, you should listen to them.