It reflects the reality of the world.

Say the UN security council ordered Russia to immediately withdraw from Ukraine. Great, right?

Then Russia just says "No."

Ok now what? What can anyone do when a country that has nuclear weapons refuses to comply with an order from the UN? I guess they could write another strongly worded letter which would also be ignored.

The nuclear powers can unilaterally veto anything because that's simply the reality of the world. If you have the UN trying to enforce something that a nuclear power doesn't want that could lead to a nuclear war. Which would be bad.

If countries are satisfied with writing resolutions that accomplish nothing then it's better than having a war. And that's the ultimate goal of the UN, to make countries waste time on UN resolutions that accomplish nothing instead of having wars.

Don't think of the UN as a police force, they aren't. Think of it more like a global politics version of lemmy. A lot of idiots writing angry things at one another and getting nothing done. But getting nothing done is better than having a war.