The thing is that someone who is listing their services on TaskRabbit wants to exchange their time for money. If it's an amount of money that's worth it to you, it's not you who valued their time less then your own, it's they who did it. Why do law firms have janitors? Because lawyers' time is economically, if not morally, more expensive than janitors' time.

It's not a cultural failure to hire someone to do something you could do yourself, but rather wouldn't. Someone who is on TaskRabbit delivering groceries probably isn't using a bicycle, can get better at it than you can by combining deliveries, and can make money at the same time. It's a win for both parties.