Unhealthy over reliance on TCAS going on here (similar to the unhealthy over reliance on see and avoid!).
TCAS will NOT display ALL of the traffic around you. It selectively picks up and drops off various paints in busy traffic environments. On the 744 into LAX years ago I remember getting a TA on traffic that just 'appeared' as a TA- IE, we couldn't develop the picture before hand because it wasn't being fed to us.
Aviation safety should be about layered defences. TCAS is an error management strategy, not a threat management strategy at the first level. It is the LAST line of defence. Heaven help us when it is the first also!