Why OAuth API Keys and Secrets Aren't Safe in Mobile Apps

I’m facing a dilemma here. I’m trying to support Login with Twitter through Restful API’s flow. All tutorials that we see online either guide you on how to implement it on Browser, or how to implement it on Mobile without any mention of backend systems. On the latter guidelines, client key and secrets seem to be saved on the App. How is it possible to implement Sign in with Twitter on Mobile app when you shouldn’t save those keys?