Factories won't build themselves. It doesn't matter if you're Google / Facebook buying servers, Tesla building factories, O buying houses, or even Disney's entirely virtual intellectual property (Marvel / Mickey Mouse / ownership of cartoon characters + movie characters). Shares are a share of the company. Owners of those shares are literally the owners of the servers, factories, or houses (or other "assets") that these companies own.
To build new assets usually requires money, and that money was provided at some time by an IPO raising money for a company. That's what a share fundamentally is.