That usually depends on the georphical density of deliveries.
And cars are faster. So that means more deliveries.
So order aggregators should have better economics.
This should allow them to take some fees while still offering a better deal.
Why isn't that the case ?
The furtest one away from me is about 10-15 minutes by bike. And that's at a comfortable pace.
Deliveroo (delivery by bike) is already quite popular here.
The thing with that strategy is, you can only do it once, using your initial capital. After you've set your new prices, new players are free to enter the market and compete with you on cost, driving you back out of profitability.
Take for example a Domino's here in Arnhem:
https://www.google.nl/maps/place/Domino's+Pizza+Arnhem/@51.9...
That outlet is in the middle of a residential area (with a lot of flats). On the map I can see dozens of pathways that cut off a large amount of distance travelled. In this case a bike (or scooter) is going to be much faster than a car.
If you're in the countryside then the math is totally different.
Even pedal-power bikes are usually faster. And these bikes are probably 500W nominal, which is already 5 times as powerful as a typical human rider. US limits top speed to 20mph, sure, but you can still peg that pretty easily, even up hills.