Being that you may have just created an online dating account, you are new and untainted.
To these guys, whether you have profile information or not is irrelevant when their primary initial goal is to get you to notice and think about them first.
On the contrary, you may get so much attention on dating sites because the same guys who would have ignored you in real life, are a lot more bold in sending you messages on dating sites.
Dating sites have made it easier for guys who would typically ignore a girl that they are attracted to in real life, to actually approach her through cyberspace.