A couple of ideas for you...
1. Tailor & Refocus your recruiting efforts.
If the kids that are joining just aren't interested in the robot and no matter what you do, they won't take an interest, then maybe your team could work a little harder on finding the kids who ARE interested in building a robot. Its often difficult to make robotics look "cool"... though it sounds like your team may have gone overboard on that... the team is "cool", but the robot part just isnt interesting. Many teams have students that never touch the robot, and IMO thats ok. We've had several kids leave our team to major in graphic design or business or things like that, all of which are important to technology companies, but yes, not the focus of a Robotics Competition. So its good to have SOME students like that, but as you say, it sounds like you are overloaded with them. In my mind a 75%/25% balance would be good. There are several things you can try
A. Talk to Tech teachers (computer classes, technology classes, math classes, science classes). See if they can recommend/encourage anyone to join.
B. Develop a
Student Application. We generally accept everyone that applies, but it gives us the chance to weed out if necessary, and also makes sure that the applicants are serious enough to fill out a couple pages of questions.
C. Develop a
Team Handbook. We have all of our students and parents sign to say that they have read the team handbook and know what is expected of them. This sometimes gets kids to consider what they are getting into.
2. Work on your Preseason Activities
The whole goal of FIRST is to expose kids to science and technology. They may decide once they are exposed that its not for them, and thats ok, but we've had several students become interested in different aspects that they never before would have considered.
A. Do some
fun/teambuilding type activities that incorporate some science/tech knowledge. Balloon Rockets, Paper Bridges, Newspaper Towers, Bottle Rockets, Egg Drop, etc... are all fun & teambuilding, but if you provide good resources, the team can learn about structures and gasses and all sorts of science/tech type stuff at the same time.
B. Do a Mock Build Season... whether you use Legos or Vex or FTC parts, get small groups (6 or less) to build robots that compete in some sort of competition. With super small groups, everyone is forced to pitch in in some way.
C. Have subteams that meet outside of your normal team meeting, and require that every kid that travels attends at least half of a technical subteam's meetings. This ensures that they at least TRY something in preseason.
D. Do a Subteam Rotation Night. We do this every year, all 10 of our subteams each have a table/location and the mentors put together short (5-10 minute) presentations/activities that show what that preseason subteam does (hint, the more hands on/visual the better!). The students rotate through every table in groups getting a "taste" of what each subteam does and get to figure out what subteams they want to join for the preseason.
3. Force Your Build Season Subteams
We do an application each year for Build Season Subteams and the mentors sit down and sort the kids out into groups. Each group must have 1 new student, 1 returning student and 1 student familiar with CAD. We attempt to balance the rest of the group with what the kids want to do (1st or 2nd choice subeams). And try and make the groups reasonably balanced in size. This may be a bit tougher in your situation, as you don't want a bunch of dead wood... but with some good mentors/build kids, you might suck in a few kids that might not of helped before. A good example is that we have had kids join the controls subteam because they want to "decorate" the controls box. Then our mentors rope them into helping with wiring or designing the box, and then they get a little more hooked.
Overall I can definitely understand your frustration. There are always students that you sort of "wonder" why they joined a robotics team in the first place. Though it does sound like you recognize that its good to have fun, but with such a large team, you could do with having a stronger core of people working on the robot. Though Realistically I think even a lot of the bigger teams will tell you they probably have a solid core of 10 kids, and then however many others that "help out" but aren't as dedicated. Really its all about balance.
Give some of these ideas a shot and see if they help amp up the interest in working on the robot, or helping the team in more technical ways.
Good luck!