Lots of men have just had bad experiences with women so they think we're all like that and what better place to take it out then on the internet where no one knows you and where you can do it to random women lol
1) The internet is a place where one can remain anonymous, thereby reducing any risk of consequence for their actions. This allows one to "troll" if you will. A response from the victim is what they crave. Once you engage they begin to 'win', as though it's a game. Simple bullying, albeit you may never know who he/she is.
2) Bitter toward female counterparts due to a bad experience (break-up, divorce, fighting etc.).
3) They simply haven't grow n up enough yet. Remember, if the responses on this site came from professionals, you would have to pay for the service.
Many guys and yes some girls google stuff about their experiences and it takes them here. They are jaded simply and have not grown up. It is easy to place blame on someone other than yourself. Some feel power in blaming others.