Apologies if this is a naive question but I haven't had much success Googling.
I've never heard of a company going public without an investment bank's support. I assume there was a law to require this.
When did that happen?
Dropbox had the support of lots of investment banks[0]. Even Spotify is hiring banks to advise it in its direct listing, even if the banks aren't underwriting [1]. If you want to read about the roles of investment banks and other financial institutions in IPOs and the existence and history of alternatives, you might want to read some of the news coverage surrounding Spotify's choice to use a direct listing.
[0] https://www.reuters.com/article/brief-dropbox-inc-files-for-... lists about a dozen investment banks involved in the underwriting
[1] https://www.bloomberg.com/view/articles/2018-01-16/spotify-w...