Excuse after excuse - rather than admitting that they're just only interested in the sound of their own voice
Some people may only be interested in the sound of their own voice - but others really are just bad at judging breaks in conversation.
A well-managed conversation takes skill, and like all skills, takes time to learn to do it well. Some people get that training from childhood, with parents and others pointing out it's rude to interrupt, that you should wait till mummy's finished speaking and so on. It's clear from some of the posters above that not all families are like this, and some learn to shout loudest to get heatd, and it's hard to break years of habit.
At school, it's usually managed by having to put your hand up, so you can see who else wants to speak (that doesn't always mean it works well.)
At work, a well-chaired meeting should mean everyone gets a chance to have their say (it may not be an equal say, depending on the purpose of the meeting,) and interrupts are controlled. A competent chair will draw in quieter people and invite them to speak.
Informal discussions are obviously less structured, but if you're the sort of person who is good at judging breaks in conversation, you can still help guide others. "Please can you wait til Z has finished speaking," would be better than, "FFS, why don't you just stop interrupting!"
Yes, interrupting is annoying, but some people just haven't learnt to manage conversation well. Whinge about people being rude all you like, but conversation involves at least two people and if you don't accept we don't all do things in the same way, you're not going to be able to help change anything, and things will just carry on as before.