Why are Tiffen MPTV filters 4mm thick?

Tiffen filters are made using the
ColorCore Process.  This process involves the filter effect being
sandwiched in between two pieces of 2mm thick glass.  This results in a
stronger filter that is less likely to break if dropped.  It also means
that surface scratches on the glass have no affect on the filter effect. 
This is very important, especially on neutral density filters where scratching
the ND effect would result in a light leak.