But that’s just being nice to those who are automatically nice to them.

That's part of a diplomacy. Also ask yourself, why are they being nice to them, especially places being plundered by wester colonialism for centuries. For example something like the current situation in Niger where French diplomat get his credentials and visa revoked over not even pretending to be doing his job + ignoring the government he was ambassador to + inflammatory speech of Macron worsening the situation. Which leads to the curious problem of westerners, including a lot of people in this thread, having weird definition of diplomacy as dictating ultimatums to kneeling nonwhite people. USA and EU are now doing exactly this: dictating ultimatums, and are bewildered when other people don't want to hear this.

Also, it's not olny being nice: there are a lot of treaties being signed, a lot of gound prepared for further ones - this is how you gauge diplomacy.

Why don’t they do the same with those who oppose them?

You mean Russia? West is only speaking to Russia with ultimatums and loaded "propositions". I guess the revelation of how France and Germany treated the Minsk agreement was a big bucket of cold water for Russia, they stopped believing what NATO says.

They just keep doing “no u, no u, no u.”

Again, what does west expect when they aren't even engaging in a good faith? Non hostile countries hear something vastly different, as proven by the examples i cited earlier.

In that respect, other countries such as Saudi Arabia and China have better diplomacy.

Idk about Saudis, but China recently vrokered a deal between Iran and Saudis, two of the most traditionally mutually hostile countries on Earth, i would call this stellar diplomacy.