That's exactly what I don't want. I don't trust the security of my app to store user credentials.
Another reason: my app has no reason to send emails except for one thing: password resets. I don't want to set up a whole email flow just for that. By using a provider I can offload that at the same time.
+1 on those services. I went with AWS Cognito, which gives you permanently 50.000 MAU for free. A big issue with self-hosting is also Email delivery; you'd have to go to some provider to send verification Emails to have a decent inboxing rate.
Only downside is the pretty ugly default login UI of cognito, but you can style it to some extend by adding a logo and custom css.
Out sourcing credentials is critical. I'd go a step further, most apps/websites should out-source all of their personal data. Liability around personal data breaches is only increasing.
Another reason: my app has no reason to send emails except for one thing: password resets. I don't want to set up a whole email flow just for that. By using a provider I can offload that at the same time.