You ever know someone that for some reason thinks that you like some dish that you actually cannot stand and they cook it for you every time you visit them? That's what this is. Third parties are the confused aunt that makes us liver.
I'd guess the real reason we keep getting this genre, despite the fact that making an on-rails shooter is like giving Wii owners the finger, is because it's easy to do. You can take your existing shooting related game and just make the path fixed and have the player aim with the remote and, voila, product on the shelf. Pointing and shooting is one of the easiest ways to use the remote without it being a broken mess and without having to come up with a creative idea. It's like a mouse point, you know your team can pull it off without fucking it up.
Also because the Wii is underpowered compared to the other systems you can't just port your game to it. You could design a brand new game from scratch but that's a lot of work. An on-rails shooter lets you reuse ideas and since you control what the user sees you can make sure that you don't have too many enemies on screen at once or that an area is too big. In an open 3D game there are a lot of variables and you certainly can't just port your game. The on-rails shooter is the cheater way to port a game to the Wii. Its heavily scripted nature gives the dev the control to make sure the game works.
Third parties don't want to make Wii exclusive content, they want to have multiplatform releases. That isn't possible so we get half-baked spinoffs instead because that's the closest thing to a port. Making two unique full Ghost Recon games would take twice the development effort. It feels like they budgetted for one game released on three consoles. The Wii doesn't allow for that so they take the budget for porting the multiplatform game to the Wii and put it towards an exclusive Wii release. But that isn't a full-sized game budget so we get a junk title. It's like no matter what they have this cost in mind for getting some product out for all three consoles and they're not going to allow the need to make a seperate release for the Wii affect that cost.
It's like if you have three sons and you were going to buy them all a snack. All the snacks are such that $2 gets you a large sized snack and $1 gets you a small one. There is a deal that the more you buy of the same item the more you save. You can get 3 large snacks for $4 or 2 for $3. You only have $4. Two of the sons want ice cream cones which are large snacks and other does not. You get the 2 ice creams for $3 and let the remaining son get something for the remaining $1 and he has to settle for a smaller snack than his brothers. He doesn't get a large $2 snack like he would have if he played ball. That only worked if he went in on the deal. He decided to be the oddball so he got less because there was only so much snack money available. That's the Wii and I think that's why we get these stupid on-rails spinoffs. You can decide for yourself if the father or the son is being unreasonable but that's what I think the situation is.
Dead Space Extraction however shows that we don't like this. That game got great reviews but still sold like piss. The reaction from the Wii userbase was clear - "we want the real game, not some bullshit spinoff."