The surface of a single planet is finite and, thanks to technology, small. In Ian Fleming's You Only Live Twice, the CIA, regarding the Pacific as their preserve, receive military intelligence about Russia from the Japanese but do not share it with MI6. Bond, temporarily assigned to the Australian Diplomatic Service, opens negotiations with the Head of the Japanese Secret Service. It is the 1960s so these are men who were active on their respective sides during World War II.
Interstellar space is infinite and remains vast despite technology. Without FTL, imperialism is difficult or impossible. With FTL, the Terran Empire and the Merseian Riodhunate face a completely different situation from that of the Cold Warriors with their single planet. There are no easy lines of communication as from Tokyo to Melbourne to London. And, outside the Imperial borders, barbarians lurk, savages armed with nuclear weapons, ready to invade like the Huns attacking Rome.