I don't know, but it makes me stubbornly cling to my type, insisting I am nothing but an ordinary INFP .
I think many of the reasons have been noted, but I think another is "specialness", aka "I'm too special to be XXXX type (insert common forum type) like everyone else here" or "I'm too special to be just one type so I'll put a word for my type" or "I'm too different from other XXXXs, so maybe I am something else (ie. something more special)".
"Charlotte sometimes dreams a wall around herself. But it's always with love - So much love it looks like everything else. Charlotte Sometimes - So far away, glass sealed and pretty." - The Cure
Latest scientific studies show that typology can cause severe brain damage, eventually making some people hallunicate they're some kind of weird geometrically shaped hyena that chases around the individual letters of E,I,N,S,T,P,J,F in an infinite blackhole of time, where they enternally struggle to catch all the letters but continously fail - being able to capture only a few at a time. Try as you might, you maybe able to catch E today but never I, S but never N, F but never T, P but never J. Every day the cycle is always repeated, only with different letters and combinations. E today, I tomorrow, and so on. You poor souls!