Interestingly, economists (also sociologists, historians, etc.) disagree agree on what money is.
For the purposes of this topic though... I think it's sufficient that marketable securities sit under "current assets" on a balance sheet, alongside "cash & equivalents."
Say an investor buys a new share in uber for $35. Her balance sheet isn't meaningfully changed. Uber's bank account has grown by $35. I'm not saying that the accounting doesn't work out. The $35 isn't counted as revenue, and doesn't impact earnings.
Still... $35 has now appeared in one account without disappearing from another.
Again, I'm not saying the accounting is dodgy. Theoretically, when you buy a $2 ice cream then you have a $2 ice cream and 2 fewer dollars. Your "balance sheet" nets the same amount. Same thing when a woodshop buys timber. But for most practical purposes in the modern economy, a company's "total assets" don't matter much. "Current assets" do.
Whether or not you want that money (or even currency) creation is interesting, but it works the same as money in practice. Gold, sovereign notes, bank bonds... Same result as shares.
Shares can be overpriced, but so can currency. It's besides the point, imo.
For an analogy, neoclassical-adjacent economics' take on international trade practices like dumping or export subsidies is pretty much what you said. EG, if China subsidizes toothbrush exports, the rest of the world gets subsidized toothbrushes at chinese taxpayers' expense. A similar point to the one you are making.
This argument is typically to counter the claim that export subsidies unfairly kill local production, and/or forces a subsidy war.
Riffing on David Graeber (the polar extreme, and not exactly an economist) somewhat... I'd interpret Uber's investor-subsidized losses differently. Investors haven't really lost much money so far, the stock price is fairly flat. They buy shares, and then they have those shares.
The shares themselves are a type of money which companies print. It is new money. Investor buys one share for $1. His net worth has not changed. Uber's bank balance has changed, and they can now fund another $1 of losses. As long as the share price holds, losses are offset by the new money (share) creation. It's that new money subsidizing rides.
On a less abstract note... I think it's important to remember that subsidies don't just transfer. They also create waste... which no one benefits from. For every $1 lost, consumers only gain a few pennies of value (utility in old timey economist speak) goes to consumers and/or drivers.
The whole reason that uber needs to subsidize rides is that consumers aren't consuming enough of them at "full price." That means they don't value rides at full price.