Because there is the difference between "peripheral" (even "proxy") wars, when the superpowers more or less successfully fight against local powers (even supported by other superpowers) and the "central" wars, during which superpowers directly fight each other on their core lands.
The same difference that with Russian proxies, attacking American forces with Russian-made weapons in Vietnam or Syria and Russian forces directly attacking American bases on Alaska (which could, and likely, should be done in the case of American attack against Russian military targets in Kursk region). Even if Russian government will denie their responsibility for the said attack.