A power thing I suppose... But security-wise the outside public does not always know who's being transported.
There is a protocol and depending on one's status in the food chain, anyone cannot bunk with anyone in the same limo. International politics are not about car-pooling. There are rules, compatibility codes and security basics to compose with for the security detail, let alone when foreign dignitaries are also being moved around.
Our former Prez had up to 15/20 vehicles at some point, and got criticized for it, the public thinking it was just him and his entourage. Well, no. There were foreign VIPs and their lackeys and security men as well, but it obviously cannot been advertize before the convoy hits the streets!
So the motorcade grew smaller in the end, but some compromises had to be made for the sake of public perception...