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).

Preventing oil exports and increase insurance premiums for Russia's export economy, because Western sanctions clearly are unsuccessful in destroying the Russian economy.

My post history shows that I do support Russia's self defense against U.S./NATO threats. In my opinion Ukraine entering NATO is indeed an existential threat to Russia, because since (at least) the collapse of the UDSSR the U.S. and it's vassals openly communicated and pursued the goal of regime changing Russia (+ Belarus, Georgia).