There are several good comments in here explaining cases and reasons to build instead of buy. But they are all techical and miss one critical thing: human motivation.

Nobody cares about your bussiness as much as you. The motivation behind your product/service provider is to take your money. Buying will always be a battle with the other party to get value for your money. They are strongly motivated to offer as little work for as much money as they can get away with because there is where profit is made.

The provider might even be more competent than you in building. But think about hiring a good programmer that just doesn't want to work vs a mediocre one that is highly motivated. I think everybody has come across these examples and knows the outcomes.