Probably for the same reason you never hear them refer to others as "non-Baptists", "non-Methodists". "non-Anglicans", "non-Pres-byterians", etc.

Edit: strange word filter you got here!

Protestantism by nature and origin is a revolutionary movement, and frankly they ultimately need something to rebel against. One only needs to view the works of those prominent heretics during the “Reformation” many including Luther admitted the necessity of the Catholic Church, and the fact that Rome has the Faith and all the valid Sacraments. Many of the first Protestants (once again including Luther) acknowledged the Catholic Church to be Christ’s Church, although their satanic pride made them acquire the opinion that they could just do it better… leading to the innumerable charges against the Church for falling into corruption, decadence, heresy etc… The rebellion (in this case Protestantism) has no purpose without the legitimate establishment (in this case the Catholic Church), and in denying Catholicism’s identity as “The Church” Protestants ultimately take away a huge part of the premise of their respective heresies.