Sanctions are usually considered the most powerful and extreme punitive action available that isn't outright war.
While the US has a prodigious military, it has issues with deploying it on large scales over large distances. Which is mostly just the result of a simple fact: the planet's big. The logistics involved are highly non-trivial, and the military has for the most part had little reason to seriously focus on the issue at least since the end of the Cold War (but even during it NATO was well aware of these logistical issues and tried making various plans that didn't depend upon a full scale US involvement). Any attempt by the US to stage a serious military defense in Ukraine will take probably months at least, and with that much heads up Russia could simply go into a fully offensive invasion into Ukraine long before the cavalry arrives. From what I gather from experts, it's really unlikely Russia could actually win in the span of a few months, short of a dramatic shift in the Ukrainian people's willingness to defend themselves. But the devastation could be significant, and Russian forces could be entrenched and reinforced for a lengthy conflict long before US forces would be strong enough to attempt to expel them or hold them off. And that would amplify the severity of the conflict between them, creating greater risk of a nuclear incident, or full on nuclear war.
A much more likely military defense of Ukraine would originate closer to Ukraine: within Europe. But this, again, heightens the severity of the conflict, making it a World War waiting to bloom, with multiple nuclear powers involved.
With serious domestic issues at home to deal with, some of which this crisis is only going to amplify no matter what, the general willingness of the US population to back an actual military response isn't very strong. Combined with the prospects of a major conflict with an entrenched aggressor thousands of miles away, a military response simply isn't viable.
The hope with sanctions, as the hope always is (though some debate if this is a vain and futile hope), is that the economic pressures on the economic and political elites will undercut the support for Putin's (or more generally the Russian Federation government's) actions enough to encourage a de-escalation and potential return to a pre-invasion state. For all the talk of how the US is basically an oligarchy these days, Russia is very much more one, so attacking the pockets of their elites is the most accessible and glaring pressure point and weak spot available. And the US has immense power in international financial markets and banking to enable such attacks. Whether it will work or not is another matter.