Because every "out of true" item in the "chain" of the mounting system will create an imperfection in the alignment of the rings themselves.
Imagine two rings, perfectly round and in alignment, squeezing the tube of the scope. No ringdings would be left on the scope because everything is pressing flatly on the scope.
If you take those same rings and put them on a mishapen action or rail, those rings will be minutely (thousandths of inches) out of alignment. This will create "hot spots" where the rings are squeezing the tube "out of true."
Similarly, if everything is in spec, except the placement of the ring crossbolts, those crossbolts will exert influence on the way the rings sit on the rail, which in turn will generate hot spots.
Imagine wearing someone else's shoes. Notice the hot spots where your feet are being "rubbed" too much because the shoes are broken in for someone else. You're likely to get blisters in these hot spots caused by misalignment of your foot to the contours of the shoe.
Every item in the ring, scope, bolts, action combination is a variable and those variables can cancel out or stack up to different degrees to create a misalignment. That will cause the rings to squeeze the tube unevenly which will mar the finish or dent the tube.