If you can't let your guard down with one of your closest friends, offering some fun for you, then who can you let your guard down with? Bev & Deanna would have no reason to suspect Will would introduce them to something dangerous. That's what was so pernicious about it. Even Picard, who is arguably the most socially isolated person on the ship fell prey, because there was at least one person there (Bev) with whom he lets his guard down. The game spread (& rapidly enough to go unnoticed until it was too late) because of the chain of personal trust that runs through the ship
On its own, that's true, and fair enough.
However, they work in an environment where any of their 'closest friends' can and sometimes do turn out to be possessed by an alien or parasite, or under some other form of influence, or even be a flat out 'double' of the original -- I'd guess about once per year. You'd think they'd have gotten a bit more reserved and wary over the years
