The main difference between the Romans and the Greeks is that the Greeks had a lot of science, but little engineering. They were astronomers, but they didn�t built aqueducts. The Romans were the other way around, they did lots of building and practical engineering, but they didn�t go any farther into science than they had to in order to make the project at hand work.
In addition to the above, you have to understand the nature of ancient societies and their system of beliefs. It wasn�t that long ago that they people of the world feared to go beyond the horizon of the sea as they believed they would fall off the earth and plunge into their version of hell. Also, a lot of society was based around pagan beliefs and philosophy. In ancient times travelling by road and sea was precarious, let alone by air � for which you might have been stoned or crucified anyway. Note the warnings from the story of Icarus who flew too close to the sun. Aeronautics although explored was not really fully studied in the recorded history of the West until people like Da Vinci started drawing the ornithopter, originally created in myth by Daedulus (father of Icarus).
The fact that they or the Romans never achieved going into space, nor the means to defy gravity (except with artillery and projectiles) does not mean that they didn�t sow the seeds and people like Archimedes who created the foundations which led to the eventual achievements in our own lifetime. And some of the rules and mathematic genius created by Archimedes are still relevant and juxtapose to principles we use in space exploration, astrodynamics and aeronautics today.
You could equally ask why the Victorians or Elizabethans didn�t achieve it, since the glider and ornithopter wasn�t re-explored as a useful gadget until the late 1800�s at tne time of the industrial revolution. Perhaps it comes down to the discovery and manufacture of new materials such as liquid hydrogen,