My supermarket offers 4 hour delivery windows that you book about a week in advance, and it's perfect (other than the occasional dodgy swap or missing item). I can work with that. But amazon (for example) offers same/next day delivery, and I'd estimate maybe 10% of my orders are late. That doesn't work if I'm relying on them for food
Now, admittedly if they were employees, they would be paid by the hour whether there were orders or not, but then both their salary and the number of couriers on duty would have to be adjusted accordingly, which means the majority of couriers would lose this job… Not a simple matter, IMO, I think there is a gap in legislation when it comes to this type of work.
Most on these ”can earn so many thousands per month” seem to follow same formula 1) Assumes that somebody works >12h per day, 6-7 days per week and 2) Mixes salary and self-employed hourly rate