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.