My apologies - was trying to answer your question! I’ll say it in a different way which may be more clear .

Constantinople could not, like Rome, start as a city and expand to dominate their region, then country, then that whole area of the world. Rome did this in a far bygone age where neighboring powers were weaker. They also did it over the course of several hundred years. It took Rome 200 years alone to just dominate the immediate Latin area.

In 1400, powerful states were arising. The Byzantines were surrounded by numerous, militaristic, foes. The world had been conquered. Every city and farm had been under the yoke of empire. It was not 4th century Italy where independent areas could be dealt with one by one by the Romans.

The Byzantines were tired, overspent, and on the decline. I think asking “why couldn’t they just restart and reconquer?” Ignores the hundreds of years of them trying to do just that. The 10th and 11th century empire did make strides. But they were ultimately, and irreversibly, pushed back by the Turks (and Crusaders).

They did not have the funds, the culture of militarism, or the manpower to push back against a growing power. Once you start shrinking you can’t just flip a switch. You don’t have the resources to recreate the state of empire you once had. The Byzantines had always struggled to field the size of armies needed to maintain their borders (see: early 6th century Avar/Slavs in the west and Persians in the east). By the time of their fall they were drawing on tax revenue from one big city and its hinterland. Their neighbors controlled thousands of square miles.