Some guys just like to go alone because their friends hold them back/aren't as adventurous as they are/are in serious relationships/injured.
There are countless reasons.
Most Helpful Girl
Most women go to clubs to socialize with friends (dance, drink, etc.). When they see/meet a guy who is out by himself, they think he's just there to try to get his dick wet, and that comes across as desperate or creepy (and sometimes manipulative, for example, if the guy is a "PUA"). Even if a woman is open to hooking up with a guy at a club, its more of an after thought (she happened to meet a guy, had a good time with him, and decided to hook up with him), rather than her sole purpose for going to the club. If a guy is by himself, he's likely going up to woman after woman, trying to find one who will go home with him, and that makes him really unattractive to women. If you're hoping to meet a guy, you want to meet one who is actually into you, not one who treats you as part of a "numbers game" and you just happen to be the first woman to say "yes".
Guys who go to bars (rather than clubs) are a little less creepy because they're a lower key atmosphere, so its easier to imagine that maybe he's on a work trip and decided to grab a drink before going to his hotel, or maybe he's new to town and looking to meet new people. But many women will still find you creepy if you seem like you're just there to get laid.0