The best answer I can give you is that you've not talked to the right companies. Spring Boot is widely adopted. As I mentioned in another thread, there are many well known and respected companies across all industries using Spring Boot.
I can also offer that Spring Boot is relatively new (only a few years old) and that it's not always easy to retrofit an existing Spring app to be based on Spring Boot (not impossible, but not necessarily easy as I've already mentioned in another thread). There were plenty of Spring applications written before Spring Boot and plenty of teams who haven't been brought up to speed on Spring Boot yet. But that doesn't mean that Spring Boot isn't widely adopted. It just means that those projects and teams haven't adopted it yet.
I've enjoyed and learned a lot from your other Spring In Action books.
posted 3 years ago
All that said, if the decision has been made to use Spring on a project, then the question of whether or not to use Spring Boot is pretty simple. In short: Why would you want to write and maintain code that configures your components in Spring if you don't have to?
There's a significant amount of Spring configuration that is common and quite similar between almost all Spring applications. Taking Spring MVC as an example, everyone using Spring MVC has to *at least* configure DispatcherServlet in web.xml and has to create a configuration class that's annotated with @EnableWebMvc (or alternatively an XML file with <mvc:annotation-driven/>) before they can even write a controller to handle a request. And that same kind of common configuration is found in all areas of Spring, not just MVC.
So the question is: If everyone has to write that configuration, then...why does everyone have to write that configuration?!?!? Can't Spring just *know* that I need that stuff? Why must I write so much common configuration to satisfy the framework? Why can't the first line of code I write be something that directly addresses the functionality of the app?
And that's the question that Spring Boot answers. Auto-configuration determines what common stuff your app will need and will automatically make it available for you. In the Spring MVC example, that means the first line of code you write is for a controller that handles a web request. You do not have to configure DispatcherServlet and you do not have to create a configuration file that enables Spring MVC. You just write a controller and Spring Boot takes care of the rest.