Recently, I saw pictures like these roaming around the Net. First is of Aragorn portrayed in the 1978 Ralph Bakshi animated adaptation of Lord of the Rings. The common caption: look, no pants (oh, and that's Boromir on the right).
And Boromir from the same movie again, doing a kung fu kick, showing the expected.
People today wonder, what the hell were they thinking back then? Of course, people should remember that sensibilities and outlooks change over time. Perhaps these observations will explain those.
1. The Barbarian Aesthetic (if you could call it that) ruled in those days. The artwork of Frank Frazetta and Boris Vallejo showed men with oily bulging muscles clad in loincloths, alongside women in bikini-type costume. No-pants was the contemporary perception of the time about ancient costume. Look at Ben-Hur and other movies about ancient times. It's only in more recent times that American moviemakers gave more thought to more "modest" costume.
2. Animation-wise, it was easier to draw. Back then, animation was hand-drawn, so it was slow and painstaking work without the help of computers. Of course, to simplify things makes them easier to draw and thus quicker to finish, so both Boromir's and Aragorn's short tunics serve that purpose.
Of course, these are not the only reasons. Other will wonder if the above portrayals are even close to the source material. Ah, that's another thing - moviemakers are fond of reinterpretation. Like what they did with Street Fighter 2 and Transformers. Some work, some don't. Sometimes, the reinterpretation just ruins it, so it's better to stick to the source material. I just hope moviemakers these days have a bit more humility so they don't go through with disastrous reinterpretations.