I don't know about the typical HN contributor but I personally lack the cash to but all the stocks in the S&P. There are 503 stocks tracked in the S&P 500 index. It would cost about 2.8 million USD to buy 100 shares (one board lot) of each if you were naive enough to weight your purchases that way. If you were to weight the stocks differently (eg. total market capitalization of each company) the amount would be higher.
Or, I can pick up 100 shares of an index ETF for a few thousand and have someone else do all the work for me including rebalancing and doing all the other required calculations (lot tracking and cost basis calculations etc.).
Trading in lots of 100 hasn't been required since I dunno, the 90s?
Assuming you're in the US there are several competent brokers that sell fractional shares. Any broker will do lot tracking and cost basis calculatioms for you, they're required to.
Rebalancing might be a pain, yes. I'd bet the drift isn't too bad most of the time, but it's probably effort every time you add or remove money. You'd want to build a tool to tell you how to add and remove to get closer to the index. If you can get the index weights and your holdings in a machine readable format, it would seem pretty tractable, but it would take time to setup; there's a reason funds have expenses, but index fund expenses are small.
I'm 100% invested in funds because it's a lot less work, but if you felt strongly about excluding certain stocks, I think it's pretty doable for say S&P 500. Tracking a total market index, or an international index would be more challenging. Bond indexes are also challenging to track, even for bond funds.
Interactive brokers offers fractional shares I'm sure other brokers do as well.