Explanation: The top photo is of a coin of the Principate era, the Roman Empire as we think of it. The bottom photo is of a coin in the Byzantine Empire. Artistic standards... dropped a bit.
One of the historians on AskHistorians claimed that they still could do realism in the medieval era, it just was out of style. It's hard to believe a fashion could last 800 years like that, but then again it doesn't take too much practice to draw better than this or any number of manuscripts.
There's a mixture of factors at play there. By the High Medieval era, absolutely there was the capacity to do realistic art - or at least more realistic than was popular. But for much of the 'Dark Ages', the capacity was genuinely lost - not because "Medieval People Dumb", but because in much of Europe, specialists and 'tribal' knowledge (knowledge passed down from specialist to specialist, not contained in the general population) of artistic techniques, especially in painting, which lacks other applications that keep tradesmen in practice, were lacking. The division of labor and interconnectivity of courts and urban centers in Europe simply was not great enough to sustain them on the same level of the patronage-obsessed Classical era - not even in the Byzantine Empire - and certainly not in art of depictions of people.
One of the most telling examples, I think, is how often Byzantines praised their own icons as 'realistic' and 'lifelike', claiming to be able to recognize the saints in visions because of them - yet we would not regard Byzantine portraiture, especially in the Dark Ages, as particularly lifelike, especially after the iconoclast controversies wiped out a generation of 'tribal' knowledge. To the people of the era, it was not "intentionally unrealistic", it was striking, like how we used to praise the 'lifelike' graphics of AAA games in the early 2000s, lmao.
I think one of the greatest examples of this lacking knowledge in the Medieval period is a kind of counter-example - the short-lived Carolingian Renaissance. While there are better and worse examples, as one might expect from a sudden and sharp increase in interconnectivity and patronage of the arts, and one can quarrel over whether it is as good as the height of Roman Imperial art, there's no denying that it's damn skillful. The Carolingian Renaissance lasted for part of the 8th and 9th centuries.
Yet with the decline of the Carolingian Empire into the era of the Holy Roman Empire and the Kingdom of France, this ability declines and disappears for another 3-4 centuries. The eras of Ottonian Art and 'Romanesque' Art in Europe are very real declines in the preservation of this 'tribal' artistic knowledge, not just a change in tastes.
By the early 14th century, though, you start to see a divide in what is stylistically popular and what is possible by the techniques and talents of extant artists with the height of the Gothic Art period, and by this time the vibrancy and prosperity of European cities and courts has developed enough so that you're essentially looking at questions of "Realism vs. Stylization" as to why things looks different than in the Classical era.
Civilization is fragile. Artistic civilization, doubly so.
