I don't know about every buy-write index investments, but BXM specifically is done with essentially ATM (technically the very first strike OTM I believe) calls against the long position, then held to expiration and cash settled. In a long bull market like the present day, this approach will always underperform the market while having reduced volatility. It should overperform the market in down or sideways markets. Also, volatility induces drag so in a compounded return, all else being equal, lower volatility will yield higher returns.

Diversifying amongst uncorrelated products is an important missing feature to this strategy for the purposes of reducing volatility. If only considering writing covered calls/puts, I'd personally prefer to reduce volatility through diversification, and sell further OTM options to reduce basis so I keep more of the directional risk in each individual position and have lower transactional costs.

Also, BMX holds contracts to expiration rather than benefit from cyclicality in price and implied volatility by closing/rolling options early when they move in your favor or scaling into positions during volatility expansions.