The interesting part about reading ancient works is they don't use the same standards we do today. In fact, they don't really have standards. For example, Herodotus intended the topic of his book to be the Greco-Persian Wars, but ultimately he just wrote about whatever the fuck he wanted and even committed a large portion to Egypt. If anyone wrote like that nowadays, they'd be completely slammed for straying from the point or being loquacious.

Philosophy is like that too. The format of philosophical works change depending on the era. A great example is Thomas Aquinas' Summa Theologica. He came out of the scholastic movement, which heavily depends upon debate. The entire work is something like this:

Question (e.g. Is God everywhere?)
-Negative answer 1 (e.g. "God isn't everywhere because...")
-Negative answer 2 (e.g. "He also isn't everywhere for this reason too...")
-Negative answer 3 (e.g. "This is also a good reason why he isn't everywhere...")
-Quote from the Bible or some church father that proves the positive answer
-Long explanation as to why the positive answer must be right
-Reply to negative answer 1 (e.g. "Your argument is unsound because...")
-Reply to negative answer 2 (e.g. "This too is wrong because...")
-Reply to negative answer 3 (e.g. "Your reasoning is false because...")

And that's how they debated in the schools, so he wrote from what he knew. Plato also wrote from what he knew, and his form is called the dialectic method that was espoused by Socrates. The idea is two or more people with opposing points of view muddle their way through reasoned arguments to finally reach the truth of the matter. In theory it sounds great, but when you say the word "dialogue," you're expecting two participants. Most of Plato's works involving Socrates is something like this:

Socrates: "Blah, blah, blah, blah, blah. Don't you think so?"
Some dude: "Yes, I think so."
Socrates: "Blah, blah, blah, blah, blah. Wouldn't you agree?"
Some dude: "Most certainly."
Socrates: "Blah, blah, blah, blah, blah. Isn't that true?"
Some dude: "I couldn't have phrased it better myself."

That's not a dialogue. That's just Socrates talking and the dude is a sycophant. Admittedly the Socratic method is starting off by asking the dude his opinion... only to shoot him down and completely trample over him for the rest of the work. I know Plato loved Socrates and all, but did you even fucking try. It's me listening to a speech. And whilst I know because this is Plato's work so it'll be Plato's opinions, then you just have to ask why bother with the dialogue at all and just write down your opinions like Aristotle did?