Because the index needs accuracy. If a company is 1-2% of the total US market cap and not included in the index, then the index is wrong right now. The longer this company is not in the index, the longer this error compounds.

In the coming few months, multiple giga-cap companies (SpaceX, OpenAI, Anthropic) are all planning to IPO. These companies will likely never meet S&P profitability inclusion criteria for the next 5 years. These are not bad companies, but because the S&P inclusion criteria were written for old GAAP profitable companies, and not high-growth companies that invest their cashflow into company growth over profits. Excluding some of the most civilization changing companies from the benchmark means the benchmark is doing a terrible job.

"Because the index needs accuracy.", and I would argue that include price accuracy not just inclusion accuracy. The S&P is a benchmark that is designed to reflect a subset of the market, and giving only some companies early access to the benchmark changes the benchmark. So if you want a benchmark that's designed to include all the big stocks regardless of age, profitability, etc then go make a new benchmark. The only thing you need to do is convince others to use your benchmark.

> Because the index needs accuracy

So you are saying that S&P 500 should be merged with Russell 2000 or rather just become a fully market index to be more "accurate". You do know that's something that exists already, having different indexes makes perfect sense and consumers can pick the ones they want based on their risk profile and preferences.

> most civilization changing companies from the benchmark

It took Google 2 years to get into S&P 500. For Microslop it was 8 years (!). So what's new?

There are indexes which explicitly try to capture the entire market- the Russell 3000 is most prominent, but the Wiltshire 5000 is another one, and Vanguard's Total Market Funds and ETF follow the CRSP US Total Market Index. I believe all of them plan to include SpaceX/OpenAI etc. within a few weeks of its listing, which is what I'd expect from their goal of tracking the total market. Other indexes follow just a few stocks- most famously, the Dow Jones Industrial Average (built during an era of when it had to be calculated by hand every night) looks at just 30 stocks in a weird way(1).

The S&P 500 isn't either of those. It has a list of criteria for inclusion, one of which is profitability. They are sticking with that criteria. If you don't like it, sell your VOO and buy VTI instead.

1: It is essentially impossible to build an index that tracks the DJIA because, since it was done on pencil and paper, it isn't actually market-cap weighted, but is share price weighted, with a correction factor for each stock to account for splits, one stock replacing another, etc. Because of that nature, the weights of the DJIA change minute by minute, so someone attempting to track it would be subject to enormous error.

> If a company is 1-2% of the total US market cap and not included in the index, then the index is wrong right now.

To be clear, S&P 500 relies on float, not total US Market Cap, and Space X will have a tiny float.

Even if it was included, SpaceX would not account for 1-2% of the S&P 500 (more like 0.1%), so even if we reason on the basis of a benchmark, it's not a meaningful difference.

But it is not 1-2% of the total US market cap, is it?

It aspires to be that way. The market decides, and it hasn’t decided yet.

Am I missing something?

> Because the index needs accuracy.

No, it doesn't. At least, not the way you are probably defining it.

This sounds to me like you may be trying to use the index for something it's not really meant to be used for.

> If a company is 1-2% of the total US market cap

Over what time horizon should that number be computed? Every day? Every second? Every month/quarter?

It is not as simple as it seems.

Maybe you know this already, but this reads like exactly the kind of reasoning that people looking back at irrational market euphorias point to as a sign things were about to go awry.