Why didn't they choose real sith names? I thought one's "Darth" name was supposed to be meaningful and symbolic of something, so why did they just use their regular human/jedi names?
Revan and Malak were not their true names, either. Revan's name is, at this time, lost to history. When he saw a vision of the massacre on Cathar, he gave up his old name and took on the identity of Revan as a symbol of his quest to end the Mandalorian Wars and restore peace. Malak was once a Jedi no-one by the name of Alek. What started as an alias became a fresh start for Alek, and he decided that everyone would know the name of Malak. The fact that they kept the names they had already taken on during the war is a symbol that their goals remain the same. Revan wants peace through power, and Malak wants power and glory. The only difference is that with the title of Darth now, they've sworn themselves to the Sith and have a new path to their goals.
Both names date to the Mandalorian Wars- and the suggestion has been made before that Revan was basically a Sith by Malachor anyway.
To further clarify, the KotOR comics contain the precise scene where the Revanchist declares that he is now Revan, and shall not remove the mask (taken from a dead Mandalorian who tried to prevent the massacre of the Cathar) until the Mandalorians are defeated. KotOR II references Revan's highly aggressive, scorched-earth tactics and how he attempted to convert the Jedi to the dark on a massive scale. Revan was already a Sith by the end of the war, he just hadn't made it Facebook Official yet. As for Malak, dear old Alek was gradually driven to become more and more extreme as the Mandalorian Wars destroyed or severed him from everything he cared about- he gradually adopted the Malak identity as a means of severing his ties with who he once was.
I don't really view it as lazy, since they were made to be Sith anyway, and it's known that neither one of them is their true name. Before the Revan novel, you could make the argument that they both thought they were doing the right thing, so there was no need to erase their past self, as there was for Anakin and Vader. Now that we know it was a form of brainwashing, though... I still choose to view it as the past interpretation though, since apparently Revan and Malak were able to break free of a considerable amount of the Emperor's influence.
I don't even consider the Revan novel worth the paper its printed on. For several reasons that I'm not going into here. That being said, since we lack a name that actually fits Revan, your best bet is just to assume that his actual name is whatever your KOTOR 1 characters name was, pre-reveal. For all we know, the Jedi Council gave Revan back his original name (note, this only works for the Bioware provided names, not random names people come up with).
For what it's worth, in my most recent KOTOR playthrough I named Revan "Vaner". Seemed to fit Star War's fetish for naming children after their recent ancestors. And I find the idea of the Jedi Order picking such an obvious name amusing.