You need to look at the world probabilistically and over an extended period of time, not just try to guess what will immediately happen and go with that. The United States is still longing for global hegemony, when it's increasingly clear such is no longer possible. That creates a very dangerous situation, especially when we seem to keep picking rather poor leaders who lack much in the way of competence or foresight.
There's also simply misreading other leaders' intentions. If the US does not believe that Putin would respond to a conventional attack with nuclear weapons, then that also dramatically increases the chances of such an attack. And in fact one could argue that we're currently trying to probe how Russia would respond to slowly escalating types of provocations.
There's also the future. As we enter into a multipolar world it's possible the dollar could collapse, which could create unimaginable chaos. Russia could have some degree of turmoil once Putin permanently leaves office, which could be seen as an opportunity for the US. We might even develop a means of completely mitigating nuclear weapons, and so on. And so Russia is going to be looking at scenarios probabilistically. You're absolutely right the chances of the US trying to directly invade are extremely low, but they are also nowhere near 0. And that's in the current state. Who knows what the future holds? And once a country is right there, in NATO, it's there 'forever'.
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Like try to take yourself back to WW1. Tensions were of course very high, but try to imagine what you'd think would happen if a Bosnian Serb (with secret state backing) assassinated an Austro-Hungarian royal. Would you expect to see Brits killing Germans over it, and tens of millions of people being killed? It just seems like a highly improbable outcome, to say the least. And in fact we could be living in a timeline where some 0.00001% probability event played out, and has completely shaped our world ever since. Such things can, and will, continue to happen - history makes this clear.