I'll try to provide a partial answer as I do think this is an interesting question about philosophy.
Reasons Philosophy is Hard to Understand
First, I would say that you might be losing something in describing the works of philosophers as "opinions". On a certain trivial level, they are opinions, but on this trivial level so is C The Programming Language, the weather forecast, and the dictionary. Presumably, you mean something more negative -- i.e., that philosophy is composed of mere opinions about which people can just choose to disagree. And on a larger scale, that's probably got some validity. But on the smaller scale, good philosophers produce relatively coherent systems that make internal sense.
Second, the task of understanding philosophy is somewhat like understanding the spec for a programming language, there are going to be some points of obscurity inspired by what we don't understand in what the author is saying. I remember when I was 12 or so that I didn't understand the programming concept array, and even when I read about it, I had trouble fathoming what they were. It didn't help that the language I started with was GW-BASIC. My point here is that for some things, we might not know the terms because we're unfamiliar with the terms people use to solve and speak about a particular problem in philosophy (and because this is philosophy, the terms are often idiolectic).
Third, a lot of philosophy refers to obscured events in history or in language. Using contemporary examples, the word "copy" has for me largely changed meanings to mean CCing in an e-mail. Phrases like LOL, IMO, FTW, and others have appeared in our language, and will either eventually become completely acceptable or become obscure enough that future generations wouldn't understand them if they read them. Hegel, for instance, cares a lot about plays that we haven't heard of. The sort of history everyone in Aristotle's time knew is something you would need a specialist to decipher now. Some of the difficulty in understanding philosophy comes from this.
Fourth, some of the philosophers are bad authors -- or at least bad authors by the standards of our times. Contemporary philosophy aims to be clear and concise, but philosophy around the era of Kant aimed to be complex. Medieval Christian philosophy is a giant system built around answers to a set of questions first answered by Bernard Magnus -- it'd be nearly impossible to understand why Henry of Ghent is bothering to word something the way he is without a basic grasp of that.
Reasons People care to bother trying
The most trivial reason is that it's considered at many schools part of a university-level education, either to familiarize people with the great "canonical" thinkers of the past or to teach analytic skills related either to reading hard texts or logic.
More substantively, we still bother reading (some of ) these authors, because they developed interesting systems that have insights for contemporary problems, or at least raise arguments that still pose challenges. I'll give two famous philosophers that have few true contemporary followers: Plato and Descartes. But people responding to them and coming in their wake do have contemporary followers: Aristotle, Hume, Kant, Hegel, Mill. It helps to understand the thought of the former to make sense of what the latter are responding to and to understand the problems they took seriously.
So for instance, there are many contemporary ethicists with a generally Kantian persuasion in their approach. Which means they need to address the problems that Kant faced and explain how their view overcomes them. Part of that task is deciphering Kant's prose. (In answer to why Kant, there's two pieces of that puzzle: (1) it may partially be luck that his is the view of a group of similar ones that people baptized into the canon but (2) he offers one of the more promising approaches to a universal non-arbitrary approach to morality that doesn't center on God (or at least not directly)).
A third reason is that some of the people who post questions are as you suggest, not the brightest or most diligent in their efforts. Some of them cannot write nearly as clearly in English as you or I. We get probably a question each week on the difference between validity and soundness -- which is basically about definitions.
To wit, you could have found lots of explanations by googling "why study philosophy" (https://sites.google.com/site/whystudyphilosophy/ http://www.theatlantic.com/education/archive/2014/02/why-study-philosophy-to-challenge-your-own-point-of-view/283954/ / http://users.ox.ac.uk/~worc0337/why_phil.html / https://gustavus.edu/philosophy/answers.php ) but instead you asked here.
Some of our askers ask great questions though which demonstrate a solid grasp of the subject matter in question and point to obscure issues. Or people ask questions that want comparisons or integrations of philosophical texts that are hard to access. The untrained eye cannot decipher a paragraph of Hegel (old German style of authorship, obscure method, references to history we don't study any more), but I can think of at least one user here outside of myself who can.