> everyone else around at every possible opportunity?

How exactly to become wealthy then?..

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> What happened to noblesse oblige?

Some things made noblesse oblige way harder to manifest.

Meritocracy. "I deserve what I have", versus "I got lucky have what I have" made harder to share back.

Globalization. When you use one community to produce and another to consume, and third to register a company, and owner lives in fourth it's hard to associate yourself with the community. Where exactly to give back? You won't even see those people.

Secularizarion. Though USA is still significantly religious place comparing to europe.

Easiness to move. Today you're here, tomorrow you're there in new zealand bunker.

Culture. Somehow the rich are in the people who are heavily interconnected, spend time together at the khe khe pedoisland.

Natural selection. The ones who care less about others mathematically have more advantage than those who care enough to spend resources on non-resource-aggregation activities.

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So. There is no intristic motivation to do so (with majority), there is no external motivation, and there is no repercussions of not giving back.