The same reason wars have always been fought. Access to and control of resources. The resources are the only thing that has changed of the years.
Nowadays the resources are:
MNC access to new and old markets
protecting their supply lines
control of foreign governments by various ways and means, some by committee, some by bribery and some by intrigue
Oil, Gasses and other commodities
territorial hegemony
human capital or "hearts and minds"
Control of near-earth space
Control of the seas
The US is a modern day Roman Empire/Babylon if you cut away the bullshyte and look at things objectively
The man with the gun makes the rules
Revelation 13:4
And they worshipped the dragon which gave power unto the beast: and they worshipped the beast, saying, Who is like unto the beast? who is able to make war with him?
Not saying that is what we are but interesting parallels.