As best I understand it, Uber's play was for dominance.
Consider Facebook and Google. Together they're worth over a trillion dollars. Now ask yourself: what's the combined net worth of the second-biggest search engine and the second-biggest social network? Nothing close.
In a business where being second or third place is pretty good, you can switch to profit mode as you please. But Uber's valuation only makes sense to me if it lets them dominate a market like Google or Facebook does, setting prices and terms for the industry. I think that if they switch to profit mode, people will start to value them like a normal business, which would mean a giant drop in valuation.
I presume that's why board members supported an aggressive jerk for so long: Kalanick has been undeniably good at seizing territory, and if one sets aside little things like morals and externalities and long-term consequences, one could argue that aggressive jerkiness is exactly what Uber has needed.
Honestly, this market never struck me as one where dominance was even possible, so the investors' theories never made sense to me. But that's my best guess as to why they've been allowed to keep burning money like this.