Finding the space
Sometimes I think all these people need is a little nudge to get them going, but nowadays even this little encouragement can be hard to find. It is so much easier to accept an invitation to roleplay than it is to go out and find roleplaying on one's own, especially if you are new to the hobby.
Roleplaying also tends to be an activity in which you need to gradually build up a lot of trust and make friends with people in one group or another. This makes some people feel as though it's a very cliquish hobby, and may feel snubbed even in cases where no snub was intended. There's just a lot more going on in a roleplaying storyline than there is in your average pickup group for a dungeon, and it's not always easy to fit new people in, even when you'd really like to.
Still, roleplayers can do a lot to help by simply being aware of the situation, and trying to be outgoing and invite more people to join them whenever they can. It can mean a great deal to someone when they get invited to join a group, even if they don't know what the group is about. Everyone in a roleplaying group should try to be as encouraging as possible of newcomers, and remember that a lot of roleplaying skills take time and patience to develop. It is tempting to reject someone you think is not perfectly suited to the task, but I think there's a lot to be said for giving them a chance to make mistakes and learn. A group in which people encourage one another is a group without a soul.
Mistaken first impressions
Another thing that sometimes stops people from roleplaying, even if they get an invitation to join a roleplaying group, is a sense that roleplaying is something other people do, especially if they have an impression of roleplayers as people who totally lose themselves in their stories and forget their real lives.
When I told a coworker of mine that I am a roleplayer, he rolled his eyes and said "ugh roleplaying." I asked him if he had something against roleplaying, and he told me a story about a group of roleplayers he knew who used to live as roommates in the same apartment and roleplay for long stretches of time together whenever they got the chance. Apparently they would play Dungeons and Dragons around the clock for days, surviving entirely off pizza and just crashing on the couches for a few hours whenever they felt too tired, only to get up and join in the ongoing game as soon as they woke up. To him, of course, it didn't matter that I had never even heard of any roleplayers doing such a thing, much less met any like that myself in all my time with the hobby. He met them, and he had that experience, and that formed an unchangeable impression of the hobby in his mind. He may logically know that most roleplayers aren't like that, but he'll still feel a negative reaction to it when it comes up because the mention of it brings up his own memory so clearly, regardless of what the truth actually is.
A similar thing sometimes happens when WoW gamers encounter or talk about roleplayers in WoW. Stories about negative experiences with roleplayers tend to travel faster than positive ones, and they create an impression the roleplayers are crazy fanatics even when actually only a few of them are. If they don't already know a roleplayer when they hear such a story, their impression will get fixed that way too. Some people relish the irony when a hardcore raider or PvPer says that roleplayers are weird, because so many people outside of the WoW community think those same raiders are weird and don't really give them a chance prove they are not either. It's never so simple, of course, and so much depends on the individual's strengths and weaknesses.
So I believe that when people say "I'm not a roleplayer but I have all these neat RP ideas," what they're really saying is that they have a lot of roleplaying potential but they haven't yet received the kind of support they would need to actually set their potential free. They may not know any roleplayers, or they may have some misunderstandings about roleplayers -- or, it's also possible that they just feel as though they don't have enough time to roleplay with all the other things going on in their lives and in the game. Support could be missing from people in the game, or from people in real life, but either way, more support from whoever can offer it would certainly help.
Whatever's going on, the only solution is more courage and encouragement on both sides. Players with a bit of RP potential in them have to see the value of expressing that potential, and possibly overcome some obstacles in trying to find other roleplayers. Established roleplayers, on the other hand, need to work hard to find and include newcomers, even when doing so might not be easy.