Usually because we’re “wired” to do that and to enjoy it. Attractive women attract our gaze automatically and we appreciate seeing them, and they can be hypnotizing and/or delicious to watch. And it can turn us on. Many guys are also nearly-always open (at least in theory or fantasy) to engaging with any attractive female they see. Oh also, guys are also often fairly insensitive or oblivious to how noticeable they are or how it may be making people feel when they stare. Beyond appreciation, there are also more fundamental psychological draws.