Flat design may be less distracting to you but that also means it's less clear, because there are fewer obvious demarcation.

I despise flat design, it's downright awful design, and done for looks rather than functionality.

to you

Flat design dominates for a reason—the less visually busy something is, the easier it is for users to wrap their heads around it. This gets proven again and again in user studies, the more busy and dense you make things, the more users miss stuff and get lost.

People's opinions on the ribbon specifically are obviously all subjective, but I would say the less distracting design would be the one done less for looks, rather it's a pretty utilitarian design if you pick it apart. This is an interface for productivity tools, and as such the interface should get out of your way until you need it—the ribbon just does that better IMO.

Microsoft also did this to obfuscate features, which is pretty apparent when you consider new users used to "discover" features via the menu system. I supported Office for MS in the early days, and this was a huge thing at the time. It was discussed heavily when training on new versions.

Why on earth would Microsoft want to obfuscate features? There's no way that motivation would ever make sense.

IIRC one of the main reasons Microsoft introduced the ribbon was that grouping functionality contextually helped users discover features, because people kept requesting features that already existed, but they just couldn't find. I remember there being a blog on the Microsoft developer site about the making of it that went into this.