They are probably not monopolies in the legal sense, since there are three of them with comparable market share and they also compete with the PC, which is open. I suspect there would be more pressure to do something about it if those weren't the case.

Apple sells something like 70% of phones in the USA due to network effects that might not be apparent to users in other countries - social shaming for not using iMessage. The European equivalent is WhatsApp, which the EU is forcing to open up.