Totally agree. I also see this often on forums, with people moaning that a game they just purchased only lasted for X number of hours. The other great thing about many RPGs is that there is not just one linear way through the game and story and often multiple outcomes to storylines and the games ending are possible. These really open up the game to repeat playthroughs extending the game by a lot of hours gameplay potential.
The greatest example of this has to be the PSX game Star Ocean as it contains 80 possible endings to the game and over 160 in game characters to collect into your party. There are so many different gaming possibilities in just that one game that you could continue playing it though for 100's of hours and still see new content.
I think the problem for many, as I mentioned above, is that they get put off by playing one RPG that requires boring grinding to level up and the way it is done is not rewarding and just feels like a chore. As with anything, if you play the wrong example of a genre to begin with you are going to be put off trying another.
Look at FFX. That game suffered from some of the worst Linear sections in any FF game due to them focusing too much on the visuals and how the game ran on the PS2. But get through those linear parts and it is still a great game. But those who played FFX my be put off from trying the new FFXII and that would be ashame because FFXII is one of the best RPGs made to date.
There is another aspect to RPGs many don't like, especially younger people today. They require dedication and a lot of time devoted to playing them and experiencing the story. Especially if you compared it to something like a first person shooter that can be picked up and played for 10 minutes. A lot of the youth of today have such short attention spans that they cannot even get though the start of an RPG which is teaching them the mechanics of the game.