The decisiveness of battles is a myth. In reality, the side that is able and willing to pump in more men and material over a longer period of time will win any armed confrontation, even if they lose every battle up to the very last one.

To answer your question:

The first issue is population. A Greek city state cannot field as many men as the German empire of 1914 simply because it is much smaller. Two small Greek armed forces meet on a battlefield the size of a few football fields and battle it out in a few hours. Two 1914 armed forces meet from the North Sea to the Swiss border.

The second issue is organization: in Europe, the advent of large standing armies is only made possible by advances in state organization and taxation. Only a steady income, a strong bureaucracy and dependable logistics allow a state to keep soldiers paid and fed in large numbers even during peace time. Still, most armies were professional armies, which naturally limited their size. Once a state is able to draft large parts of its population (which is a bureaucratic nightmare), arm, feed and command them in the in the field (which requires modern means of logistics, industrial production, and communication) we see these big draft armies made up of millions of men that allow states to fight an ongoing war for years along a continuous front that you probably mean when you speak of war being more centralised.

So the tl;dr is: societies became larger and better organized, which allowed them to increase the size of their armed forces, which leads to long wars on continuous fronts.

If you want to learn more about this, I recommend "The Allure of Battle" by Cathal J. Nolan, "The Face of Battle" by John Keegan and "War in European History" by Michael Howard.

No, they where just smaller, but if you think of Roman limes, and it's legions which needed to cover at least the border in Europe and middle east, and Roman so it's required a lot of garrison. For example 1k men for 1 big town or fortes, so let's say that you have like 100 along the Rhine border, so that's are 100000 men for just one border, then add the others and the necessity to preside the provinces for from border. So you need like 3/4 hundred thousand men, only to garrison duty. The same fact is true form all of the middle ages, in which kings and dukes need to preside theirs castles. And still is the some for much of the modern age, let's take example of Charles V, king of Spanish Empire. He had an standing army of more that 400'000 professional soldiers, but the lion share of his army was dedicated to garrison the innumerable forts in all the empire. And so the army he could actually move across Europe was like 30/40 thousands of men. And there was not so secondary problem, of enemies fortes to consider and the problem to equip, feed and pay all this men. The situation will change around XVIII century, when most of the European will adopt the police of fewer buy not strategically located fortresses, and therefore making available more men to actually bring on the battlefield, the best example of this transformation is the france army of king Louis XIV, and the Vaouban fortifications.

If you want to know more there's a book "The military revolution" written by Geoffrey Parker.

Not an expert but I believe in the past battles were farther between and of more significance due to lack of modern transportation, they often had to sail/walk for days/weeks/months to reach their enemies. Additionally without the internet/news/tv/radio it was harder to be constantly monitoring eachother during times of warfare.