It's the nature of 'gig work'- riders can work when they want to, and (more importantly for them) not work when they don't want to. When conditions are good (say, a nice day in the late-morning after rush hour), there are a hundred riders willing to take the job- when they're <deleted> (like torrential rain and flooded roads) there aren't as many. These riders are on a schedule as well- if their 10-minute ride in good conditions becomes a 20+ minute ride in poor conditions, they get paid the same, so their effective 'salary per hour worked' is halved.
Sometimes when it's raining hard, the wife will say, 'Call Grab'- I tell her to wait a bit for the rain to stop, because they probably aren't coming and the order will be cancelled- it's the nature of the process, and I don't get upset about it.