No it's called priming a two sided market. Do you think supply and demand grow in harmonious lockstep?
They have to ensure supply while they bring on new customers (has associated acquisition cost) if they acquire a customer and that customer isn't able to purchase anything from them due to lack of supply (or in the case of uber the price is too high because of surge kicking in) then that customer will churn forever, and potentially share their negative experience with their friends.
So basically, nail supply, grow customers, don't run out of cash before the whole thing is working.