They can defend themselves all they want, as long as they pay for it. Why should our tax dollars be sent to them? With the amount of money we have sent them, we could have ended homelessness in every part of this country.
Besides, they got themselves into this mess by deciding to be puppets for the west. Let them deal with it.
To add to what others have said, there have been some pretty clear reasons in recent years why funding Ukraine's self-defense is beneficial for other countries... beyond expressing basic human compassion, that is.
If you want to be cynical, Ukraine is a major agricultural and mineral exporter in the region. If Russia succeeded in annexing Ukraine, it would cause trouble for not just neighboring countries, but those further abroad — it's safe to presume Russia wouldn't export to democratic countries to the degree that a free Ukraine does.
And remember, Putin's goal is imperialist expansion. He insists on uniting countries he sees as 'belonging' to Russian culture, and that means invading them and spreading Russian dictatorship. He's never satisfied with just one territory or one country; he saw the lack of major pushbacks on Crimea and parts of Georgia as evidence he could steal the rest of Ukraine without much of a fight. If we let Putin take Ukraine, he'll just move on to the next country he feels he should own.
For that matter, supporting Ukraine also makes Russia less of a threat in general. It's proving that Russia's military isn't nearly as strong as Putin claims, and it's grinding away that strength on a daily basis. This is also destabilizing Russia itself; more of the population realizes that Putin is lying, and that he's willing to throw people into a meat grinder to satisfy his ambitions. Many Russians have tolerated Putin's autocracy as long as it meant living in comfort; they're less accepting when it means they or a loved one gets drafted into a war they're unlikely to win, and when economic isolation hits home.