It’s not the equivalent of $5 million dollars. It’s $5 million in an era where the average person earned $571. Converted into today’s purchasing power, that’s more like $500 million.
Or put another way - The decree said that a pound of gold was worth 50,000 danerii. A Roman villa for a wealthy nobleman would be 200,000-500,000 danerii. Call it 10 lbs of gold. So a hot air balloon made of silk would be worth the same as maybe 10-20 Roman Villas. And Villas were not ramshackle houses, they were estates that could be more than 100,000 square feet, with living quarters for the family, slaves, workers, pens for livestock, baths, in-floor heating, etc.
That’s how freakishly expensive silk was. In fact, shortly after the decree inflation drove up prices to where a pound of silk was said to be worth its weight in gold. At that point, your 250 lbs of silk would be worth 25-50 roman villas.
It is conceptually different from flying like a bird. You can’t emulate the smoke, you can’t even grab hold of it and rise with it. Even then, smoke is just blown about by the wind without control. However, some people did observe small hot air balloons and probably did conceive of the notion of carrying humans aloft. While this would not work out easily the Montgolfiers did manage to do it, and the record is slim on any earlier attempts. I don’t even know of any early myths about balloon flight.
It was feasible for someone of means to create a large hot air balloon over a thousand years ago but that doesn’t mean it was likely to happen. It takes much less inspiration and fewer resources to make stirrups for riding horses but the first such devices are only found a few hundred years BC, long after people were riding horses, and they didn’t take hold in Europe until long after that.
I think it’s also possible that the reason is a different way of thinking in Enlightenment Europe than previous/other societies. That suggestion is one a lot of modern Westerners might be uncomfortable with, because they see it as somehow ‘self’ aggrandizing, but it seems possible. Not a slam dunk mind you, since other societies had produced stuff more ‘modern’ and complicated in some ways than manned balloons. Also perhaps more practical, though maybe that’s also part of the difference in mindset, more likelihood in post Enlightenment social conditions that somebody would do it just to do it, or take a more expansive view of what could be ‘practical’ (the Montgolfiers had some idea of their invention being practical, though it didn’t necessarily turn out that way in the near term).
Anyway I don’t find the arguments that the materials were too expensive to be very convincing. Rulers of many previous societies had much more wealth than those guys, even correcting for the fact they were in the business of making the main material they used. And there wasn’t anything technologically advanced in the Montgolfier method of generating the heat, so they were not ‘standing on the shoulders’ of others nearly as much as 18th-20th century inventors who brought to fruition all sort of ideas people all over had had for ages but which needed modern metallurgy, in particular, to actually happen.
Another possibility is just random chance. Somebody might have committed to accomplish this long before, but it just happened nobody did.
I want to point out, again, that when I did that it was in response to a post where somebody asked me if I could build a silk balloon. Obviously, if I was building a balloon I would be buying material at current prices.
Note the language problem: only hydrogen (as used later in balloons) was initially identified as a new thing. They were still inventing the terms they would use to describe what they were finding.