It's on a separate network so they may go down and cause outage in your own app, go out of business, etc. (Yes, your IaaS service provider can go down as well, but that's a much larger operation and something that's very-very likely to have a much better uptime than what you could do on-site. And whatever the case is with that, it's still on top of that.)
Abstractions aren't free. (BTW, I don't see it as an abstraction, it's simply outsourcing. You'd use some kind of API anyway even if you hosted the job queue yourself.)
