Because if they already have revenue, they have numbers around who and how many people are willing to pay -- there's at least something to go on and they get valued like any other company. Investors start with that and calculate forward how much they'd be willing to pay for future revenue and growth.
The start-ups with no revenue don't have that, so they make guesses about what their market can or will be, which is often over inflated and rarely accurate, but nevertheless is the basis for how much people invest. Investors in this scenario take those numbers and then back into what they think the value should be. Or worse, they compare it to other hyper-inflated companies and arrive at a valuation via group-think.
The start-ups with no revenue don't have that, so they make guesses about what their market can or will be, which is often over inflated and rarely accurate, but nevertheless is the basis for how much people invest. Investors in this scenario take those numbers and then back into what they think the value should be. Or worse, they compare it to other hyper-inflated companies and arrive at a valuation via group-think.