It is so, because most spam filters are dumb themselves despite technical wizardry they utilize. They employ incredibly sophisticated technologies to solve a trivial problem - just like in that old anecdote about the US and Soviet space programs trying to solve the problem of zero gravity obstructing the working of conventional writing instruments (pens and ball pens). Americans spent millions of dollars to invent a ball pen that works in zero gravity. The Soviets used a pencil.
The problem of spam is that of barring unwanted visitors from entering your premises. Techno-wizards designing the spam filters try to solve it be second-guessing the character and intent of the visitors. I, however, solve it in a simple, effective and time-honored way - by locking my door and letting in only those whom I explicitly recognize. That is, all my incoming email traffic goes directly to the trash bin, except those messages whose senders match the list in my address book, or who have a "key" i.e. a pass code included in the subject line.
Wojtek