Dentists and Specialization

I had the unpleasant experience to have two root canals treatments done the last few weeks. I went from dentist to dentist.

Medical students spend 4 years in school plus 3 years as residents and come out specialists. By that it means they know (supposedly) everything about a certain part of your body. Plastic surgeons can do noses, breast implants, liposuction, the works.

So why do dentists spend 4 years in dental school and learn only one thing? Why can’t they do crowns AND orthodontics? Why can’t they learn how to clean teeth and do a root canal? Why so specialized?

I don’t remember it being like that in Brazil. If a dentist or a dental student read this, please enlighten me. Thank you!