Because that's the Italian way of doing things if you wanna get to know them you have to come by and stay for awhile eat some pasta, drink Grappa. They have to be 100% certain that you're there visiting them not for your own benefit but because you're their friend regardless of the circumstances kinda like a marriage in riches or in poverty that you'll never leave. Johnny had a memory of everything that you said and if you differed from what you had said in any detail you have a lot explaining to do. Johnny didn't need a polygraph he had other ways to get the truth outta you. Casso hired mafia cops Eppolito and Caracappa to kill Johnny however not just anybody could meet John Gotti it had to be done through mob emissaries and if you did meet him in person someone would have to speak for you, this made it hard for Eppolito and Caracappa to get close to him so they needed a contact within the Gambinos someone who would change sides against Gotti. But believe me when I tell you wouldn't leave the clubhouse if they suspected that you were a traitor. They could tell by your facial reactions, and body language or if you were to quiet and didn't wanna talk with anybody just kinda all to yourself. They'd know that something was wrong, remember Johnny said they're (FBI) gonna come after me anyway it doesn't matta what I do.