The same question may be asked of those who look to the Magisterium for guidance. (I am excluding the left wing of Roman Catholicism, which is essentially secular humanism with a religious veneer.) Were the Magisterium clear, why do conservatives and liberals struggle within the mainstream of the Catholic Church? Why do traditionalists disdain mainstream Catholic conservatives such as EWTN and Opus Dei? Looking at the traditionalists, it appears they are themselves divided into camps such as the Lefevrists, the Feeneyites, and several others and are at odds. Yet it is the traditionalists who take the statements of past Popes and church councils most seriously.
Were the Magisterium clearer than the Bible, there would be seamless unity within Roman Catholicism. No such seamless unity currently exists, though it may have in the late 19th and early 20th Centuries. Furthermore, if Tradition meant the same thing to Roman Catholics as it does to the Eastern Orthodox, there would have been no schism over 900 years ago.
I suspect that the short answer to your question would point one quickly not so much to the obscurity of the magisterium, but to the hardness of heart that we all are prone to. You and I seem to be in substantive agreement about "Catholic" leftists, but my argument applies equally to those on the right-edge, too.
I myself am a traditionalist Catholic, preferring, as a matter of clear presentation of the theology of the Mass, the so-called Tridentine usage. My general theological outlook tends to gravitate toward the older presentations of argument, as I believe they are clearer. However, inasmuch as I have no doctrinal problem with the "New Mass" of Paul VI, when it is celebrated according to the rubrics in the missal, I am *not* in substantive disagreement with orthodox Catholics of the Novus Ordo persuasion. In other words, within the umbrella of Catholic orthodoxy, there is certainly some latitude to expression. This is a similar situation to what I alluded to in my original posts about the different rites within the Church.
But, being, as I said, a traditionalist, I feel qualified to address some of the other issues from the "right-side" of the Catholic spectrum that go too far.
You lump into the Catholic mix the Lefebvrists, the Feeneyites, etc. erroneously. They are in schism. They are therefore not in union with the pope. They are therefore not Catholic. They are therefore not part of the spectrum of belief held acceptable under the umbrella of Catholic legitimacy.
These people of whom we have spoken, from the left and the right, have removed themselves from the Church. The leftists among them are classically Protestant in their disavowal of legitimate Church authority, which the rightists, in selectively rejecting the same authority, are little more than "Protestants in fiddleback vestments." I feel no responsibility to own-up to either extreme.
Meanwhile, the Church goes on proclaiming those things which it has always proclaimed. High degrees of boldness in that proclamation may sometimes be lacking these days, but denial of any doctrine, once proclaimed, has never occured. Those people who hold to that doctrine, and only those people, are Catholic. St. James, in James 2:10, is sufficient authority to substantiate that claim.