"ETFs that synthetically replicate major US equity benchmarks have clear structural advantages over physical ETF models, which has delivered outperformance and helps explain the big difference in demand.
Foreign investors in US stocks are generally subject to a withholding tax on dividends of up to 30%, although many can reduce this to 15% through the application of tax treaties. However, under US tax law, namely the HIRE Act 871m, swaps written on indices with deep and liquid futures markets, e.g. the S&P 500, are not required to pay withholding taxes on dividends.
This means that while a European-domiciled physically replicating S&P 500 ETF will generally be able to achieve a maximum of 85% of the dividend yield, a synthetic fund can theoretically achieve up to 100% of the full gross dividend amount. With the S&P 500 yielding around 1.9% (2% on average over the past decade), this exemption means synthetic funds can potentially achieve up to 30 basis points of additional performance each year."https://www.etfstrategy.com/why-investors-are-flocking-to-synthetic-sp-500-etfs-10339/