Lyft can be profitable today if they wanted to. The only reason they aren't is because they are fueling growth by propping up the supply of drivers through driver signup bonuses.

Ride sharing is generally supply constrained (meaning there's plenty of demand but drivers are in limited supply). If they stopped paying driver bonuses to expand in new markets or maintain share of drivers in mature markets, then the number of drivers would likely start plateauing or declining (due to high turnover) -> prices would increase and/or rider wait times would increase -> rider demand would fall -> growth would stall.

In short, they're choosing growth over profitability because that's what investor want to see. As soon as they choose profitability (which they might have to after IPO), their growth will come to a halt and their share price will tank. Watch.