Because it's often easier, cheaper, and more efficient in cases that mirror public needs. Alerting, SMS, cloud storage, all are solved and competitively priced. And don't get me wrong, there ARE use cases for doing certain things custom or internally. There will need to be a mix of things.

The issue, is having an appropriate SLA and having the ability to hold companies accountable when it's not met. You need stated provisions that won't happen. Most commercial enterprises already operate under this model successfully, however many of the tools don't have SLAs around an earth quake. Most companies are willing to provide those provisions but it totally will come with extra cost which is typically not budgeted or sales teams or contracting officers are not equipped to have these conversations.