The UN has a percentage of US soldiers, and soldiers from other countries that are members. US soldiers number the highest of any one group but make up less than 50% of the total end strength.
The budgt is also provided by member countries on an annual projected basis, in other words our dues are already spent whether they use the funding or not but for the us to go into Ukrain on our own we create another bill for the american people.
And as the us entering ukrain we are considered a warring or opposing force, the un has a charter for any non member pact country (ukraine) or member country (russia) to resolve conflict, disputes, etc. By having binding pacts with the un they agree to allow un forces to have access to the country for situations like this to act as peace keeping forces.
The US would simply be an invading opposing force accurately described as inciting a war