Rome built roads using several layers of various materials topped by flat paving stones, but only a few of those roads remained by the 17th Century, in bad shape where they existed at all. Most of the other "roads" were dirt paths, quickly reduced to muddy tracks during some seasons. The ability to create new "all season" roads capable of handling supply wagons in volume, or to renew the old ones that were slowly breaking apart over the centuries, was close to non-existent. Shipping supplies via road to units in the field effectively hadn't existed since Rome's collapse, rivers were useful but limited, canals only extended their scope slightly, and rails hadn't been developed yet. Basically, one sent an army, fought a battle or two, and when it ran out of supplies, it either had to take them from the surrounding populace,thereby stripping the area bare at least until the next harvest, or else break up and return home by circuitous routes in order to spread out the load on the food supply. Taking over a country by methodical advance with an army being continuously supplied and reinforced still wasn't an option at that time; you fought a series of separate battles, or short seasonal campaigns at most, over a period of a few years until one side was too exhausted or broke to continue.
In the case of the 30YW, each time one side reached the point where it could no longer field a viable force, another country would take up the cause, since they had a common objective. Nobody was willing to throw their own forces, and in particular, their own funds, into the grinder if they could help it, but whenever there was no one else capable of supporting the cause, they stepped in, in turn.