A lot of investment is about minimizing risk. If you want examples of hyped-up tech companies that failed after IPO, just look at the dotcom bubble, it wasn't that long ago.
Well if you want minimal risk don't invest. But it's not just about minimal risk but rather risk adjusted returns. Most people (outside of WSB) do not dump their entire portfolios in a single IPO.
Most people also don't have the capital or time to diversify, to make a significant number of investments into individual stocks. They are better off with an index fund.
You can put the majority of your money in an index fund and dabble in a few riskier investments with a few percentage points of your portfolio. With fractional shares even those with the smallest of portfolios can do this.