The same reason you invest in anything else? You think you can make more money from it than the alternative within your investment timeframe.
Uber's last private valuation was, what, $60 billion? How much ROI do you think the LPs in the last round want to see from that investment after a few years? That sets the lower boundary for Uber's IPO price so unless Uber does a down round (which wont bode well for investor confidence), they're going to have to deliver a hell of a lot more value for their share price than Lyft, which means their is a lot more risk attached to them than lyft.
Uber's last private valuation was, what, $60 billion? How much ROI do you think the LPs in the last round want to see from that investment after a few years? That sets the lower boundary for Uber's IPO price so unless Uber does a down round (which wont bode well for investor confidence), they're going to have to deliver a hell of a lot more value for their share price than Lyft, which means their is a lot more risk attached to them than lyft.
Buy low, sell high.