Um, perhaps because the Catholic church does not allow you the privilege.
That aside, the problem is that Scripture that is clearly and plainly stated and teaches facts, does not NEED to be *interpreted*. The problem arises when someone takes a clear passage of Scripture and tries to make it say something it doesn't by reading into it, or starting with preconceived notions/traditions and forcing Scripture to conform to their beliefs instead of conforming their beliefs to conform to Scripture.
To be more precise, RCs do not have the right to interpret Scripture according to their judgment if it conflicts with RC teaching, but within those boundaries (and Rome has infallibly defined very few verses, if at all) they have a great deal of liberty to interpret and wrest Scripture to support Rome, as they understand her.
And which (as recently seen in the case of Lumen Gentium) can vary somewhat from RC to RC, as well as the degree of extrapolation they must engage in when trying force Scripture to support traditions of men which do not depend upon Scriptural warrant.