Smart (and rich) people lawyer up regardless of their guilt or innocence. It would be of no help to finding your daughter for you to be charged as the killer (which would mean that the police would stop looking for the real killer).
Unlike you,I believe that the overwhelming majority of those charged with serious crimes are guilty.Not all of them but pretty close to that.
The very fact that you're pledging full cooperation and keeping that pledge would be noticed by the cops and the prosecutor.Volunteering to give the cops the biological samples they might want...volunteering to come down to headquarters whenever the cops want to talk...giving the cops full and unlimited access to your house,your office,your papers....not being nervous as you're being questioned would tend to make cops think you're not guilty.
And if you should be (wrongfully) charged,*then* you get the lawyer.
From one of the books I read on the case, the Ramseys each got a separate attorney. If this is true, it likely indicates that the interests of the two Ramseys would prove to be different if push came to shove in the case (for instance, if one of them were charged). That tends to make me wonder one of them did it, and the other one knew. In that case, two attorneys really makes sense.