Probably to make sure it stays that way. Logistics by ship generally has a big advantage over logistics by land. There is a rough pattern over the last century or so of the big navel empires (UK, US, Japan) having a big military advantage. In the case of the UK and US their strategic policy has a big component that involves restricting their opponents access to resources water (eg, Germany around the world wars, China in the modern era or the way the US controls the sea-based routes out of Saudi Arabia and the land routes tend to be militarily unstable).