Because typically one expect a return on investment with that level of spending. Not only have they run at a loss for years, their spending is expected to increase, with no path to profitability in sight.
There's an element of arms race between players, and the genie is out of the bottle now so have to move with it. Game theory is more driving this than economics in the short term.
Marginal gains on top of these investments probably have a ROI now (i.e. new investments from this point).
I am not saying OpenAI is Amazon but am saying I have seen this before where masses are going “oh business is bad, losses are huge, where is path to profitability…”
I do know that in the late aughts, people were writing stories about how Amazon was a charity run on behalf of the American consumer by the finance industry.