That's done because of network latency. With a centralized service like local Monero was they could offer that because if somebody took an offer for one payment method they could immediately disable all the other ones so that there was no conflict and double locking of Monero or fractional reserving of Monero. Running everything over the tor network introduces drawbacks and latency is a huge one and so they cannot guarantee that quality. If you had the same Monero earmarked for say three different payment methods and somebody took one and somebody else tried to take a different one, that Monero would not be available.