Why do people advocate buying S&P ETFs instead of a global ETF? Is there a great degree of confidence that the US stock market will continue outperforming the rest of the world in the future?

I totally understand where you are coming from, I had this question when I'm starting to invest too.

S&P500 has potentially higher returns compared to global ETF. Some people feel that the S&P500 is diversified enough. S&P500 index is commonly talked about in books, youtube, news, etc. This might be a reason why more people understand and invest in S&P500 ETF like VOO. I mean there's no right or wrong.

For me, I prefer Global ETF due to the diversification. US stock market performed well in the past but this does not mean they will continue to performing well. Investing globally will also reduce risks such as currency risk and political risk.

The core of my portfolio is in Vanguard total world stock index ETF.

1) High liquidity.
2) Highly regulated.
3) USA is the largest producing country in terms of GDP and the highest purchasing power.