Probably the military vessels needed the maneuverability. What's quite surprising to me is how long such large wood cargo vessels were built and in use. Clearly this one exceeded what most down the ages considered the practical limit. :^)
The late Lionel Casson wrote about the grain haulers of the Roman Mediterranean, they were huge, and those in the know who wanted to travel great distances by sea booked passage on those. I'd be surprised if the remains of some aren't down there on the seafloor, such as in the straits of Messina.
Fascinating. I wrote a treatise on Bible numbers. One of the numbers I dealt with was the number 276, which I identify as indicating Satan's kingdom (in the Bible.)
I mention the passage in Acts 27:37 that states that there were 276 souls on the ship carrying Paul to Rome when it shipwrecked. Showing a picture of a common merchant ship plying the Mediterranean at that time, I hypothesize that the number must represent a Biblical 'code' since the ship pictured could not possibly carry that many people plus cargo.
At your mention I read a bit about Lionel Casson. It appears I may have to eat a bit of crow - hopefully with a good fowl sauce. 😕