But no steam engines then, unless you count Hero of Alexandria's little toy!
A.
Actually, according to some controversial accounts, in the 16th Century the Spanish were claimed to have successfully held sea trials of a paddlewheel-driven ship powered by one of those highly elementary aeolopile turbine engines like Hero's designed or adapted by a clever fellow named Blanco de Garay (see:
https://en.m.wikipedia.org/wiki/Blasco_de_Garay), who also developed some sort of underwater diving gear for salvage operations. But for whatever reason, possibly political or financial resistance from within the Spanish bureaucracy, and despite the seeming favor and approval of Emperor Charles V, who promoted the inventor to a higher station in life, the steamship project never moved forward.
Certainly the Roman world had enough technological and theoretical knowhow to match or perhaps even better such 16th century copper kettle steam turbine efforts. And as even a cursory examination of the other devices described in his "Pneumatics' soon reveals--many FAR more sophisticated than his aeolopile--Hero and his predecessor and successor researchers in Alexandria had extremely developed understandings of pneumatic and mechanical principles, particularly with regards to the effect of temperature on vacuum or pressure and of siphons, valves and pressure vessels--all of which
could have led to some sort of workable steam engine, perhaps similar to Watts' or his predecessor Newcomen's in the 18th Century. It seems like all of the necessary pieces were already there in 1st Century Alexandria for this key advance towards industrialization of a practical steam engine, but no one put them together. Why not? Lack of suitable fuel? The Roman world, certainly the Byzantines, had "Greek Fire" based on naptha distillates. Other fuels were commonplace including biomass such as olive pomace, or the crushings left over after oil extraction, which could have handily and cheaply fired a steam boiler. They even had coal--not only in extremely widespread use in provincial Britannia, but in some surface deposits in northern Italy itself (Liguria) where the locals used it for heating, grain drying and even pottery making.
Everyone knows the delightful "Sippy Dippy Bird" toy, right? That was also one of Hero's devices--also found described in his "Pneumatics" along with automatically opening doors, a vending machine, twittering birds automata and other wonders operated by heat, cooling condensation, levers or valves. When I look at a sippy bird I cannot help thinking of the grasshopper style paddlewheel cranks of the 1914 Tyne River steam tugboat "Eppleton Hall" in whose afterquarters I once lived in San Francisco Harbor in the '70s (see:
https://en.m.wikipedia.org/wiki/Eppleton_Hall_(1914)), while its motive principle just seems tantalizingly kindred to Watts' energy-conserving condenser, which helped boost the efficiency of the Newcomen design steam engine towards real practicality.
In short, I'm still left only with "It just didn't happen" as the rather unsatisfying explanation as to why the first dudes on the moon weren't named something like Nigelius Brachiumfortis and Buzzius Aldrinus.
Slavery and the supposed resulting lack of need for automatization is often posited as a cause hindering Roman industrial development. But I have a hard time buying this argument alone as sufficient. Unlike the largely racially-based lifetime slavery of America and other New World colonies and their "mother countries", slavery in Ancient Rome was a rather "moveable feast" as circumstances went. Freedmen abounded. For example, when it was destroyed by the volcanic eruption of Mt. Vesuvius in 79 AD over a third of the registered citizens of Herculaneum were elevated freedmen. So, bondage was not necessarily a lifetime sentence, but might have been considered by many almost as an internship to learn the ropes as a sort of Roman-in-training under a sort of patron. And it's hard to imagine a person clever and industrious enough to earn his manumission as thereafter becoming just too torporous or weakminded to refine production processes or tools to earn even more. Freedmen tended to be fairly driven folks, making up a fair proportion of the Roman nouveau riche in all eras.
One possible partial explanation of a lack of industrialization, or at least motorized mechanization, may have been an accidental boon of geography stealing the wind from the sails of those Alexandrian theoreticians, as it were, robbing them of the Necessity said to be the Mother of Invention (which, regrettably for us Yankees, was one of the original sensible mottos found on American coinage later supplanted by the mawkish "In God We Trust"). You see, the Nile, at whose mouth at the Mediterranean Alexandria stands, is blessed with ideal winds and current for river travel and commerce: the winds tend to blow prevailingly southward, while the current alone carries craft inexorably northward. Accordingly, the local economic need for a nautical engine, a primary practical motivation for developing one at all apart from perhaps mine drainage, is all but removed from consideration.