Because Catholics and Arminian Protestants share the belief that they can effect their salvation.
This was the core of the Reformation, Mack. "Faith Alone." And it's no surprise it's not being taught today. It's no coincidence we're saturated in "do-it-yourself" theology.
Jacob Arminius, with the backing of various subversive groups, tried to turn back the Reformation by again returning to the idea that man can help in his salvation. (Thus putting the power BACK into the hands of men and the church.)
TULIP is a response to that blasphemy. Man is saved by God's gift of faith alone.
Those who want to keep the separate levels of hierarchical power intact, i.e. the Romanists, the wealthy, the powerful, the Arminians, all say the same thing -- "Go for it! You can do it! It's all up to you!!!"
Calvinists say "Salvation is of the Lord." "No King but Christ."
It does make a difference.
And Calvinists say it doesn't matter what you do. Either God chose you or not!!! Have at it!!!
Wait a second. Is that really what you believe as Calvinists? Or have I portrayed a hyperbolic image of your beliefs to more easily defeat them in debate? Say it ain't so mammy. Say it ain't so!!!