At 0.4% of GDP total US aid commitments (this includes non-military aid -- military aid is about 0.2% of GDP) are similar to the UK, but only 2/3rds that of Poland. Some people might not consider that "massive", but that's how you have defined it. Also European countries have taken in many Ukrainian refugees. That cost isn't counted in these figures.
It's misleading to look at the "sticker price" of military aid -- much of this has already been produced, and would not be replaced, but would become obsolete and be scrapped (the ideal fate of any military equipment!), so giving it to Ukraine doesn't cost the US the original procurement cost of the hardware.
Of course people don't always know how much the government spends -- be honest, what percentage of GDP did you think was going to Ukraine?
The US is spending about 5.6% of its defence budget to devastate the Russian military. From a pragmatic point of view, that is a very good deal for the US.
You believe that US aid is prolonging the war?
Firstly, ending the war is not in the hands of the US, it is in the hands of Russia.
Suppose the US had not supported Ukraine? Perhaps the war would be "over", and Ukraine subjugated, but Ukraine had defeated Russia's attempt to take Kyiv after a few weeks, before the US aid you are irritated by arrived.
More likely, the war would continue as a "frozen conflict" -- Ukraine would be unable to recover its territory, but Russia would be unable to secure it.
Henry Kissinger favours that outcome, but it will not end the war because the Ukrainians believe they can win, will not accept a settlement with a country waging a genocidal war against them, and know that any agreement will be broken by Russia if they find themselves in a stronger position in the future. See Zelensky's ten point peace plan for what would be acceptable to Ukraine.
Injustice is not a good basis for peace.
If you want the war to end quickly, then a Ukrainian offensive with enough momentum that the Russians cannot reverse it is the most likely way, and that requires more, not less US help.
The above just discusses your two opinions.
Your actual question is: Why is a Russian defeat good for the US, and so worth the US spending money on?
- Russian expansion threatens the US's NATO allies -- with Russia defeated, NATO countries become more secure. Listen to Russian nationalists' rhetoric about Poland and the Baltic states.
- Russia's friends, such as Iran and North Korea, are a threat to US security interests. Weakening Russia weakens them.
- US indifference to the fate of Ukraine would send a signal to Putin that he can expand further, and to Xi that he can have Taiwan.
- As other answers explain in more detail, the US is a signatory to the Budapest Memorandum, which guaranteed Ukraine's territorial integrity as part of its agreement to give up its nuclear weapons.
- There will be opportunities for US companies in the rebuilding of Ukraine, but I would not put much weight on this -- it is the broken windows fallacy. There would also be opportunities if the Ukrainians were spending their money on developing their country rather than reconstructing it. Eventual integration of Ukraine into the EU will be good for the world economy, as global trade will increase.