Why do editors (such as MS Office apps) have so much redundancy?

You'd probably needs someone who works at Microsoft to answer this one, but from the outside observer, there are a number of reasons why this might be the case:

• They cater for a very diverse group of users: think about the audience and users of Microsoft products and perhaps this is a way to accommodate all the different ways that people might use the product. They do allow you to also customize the ribbons and various toolbars, which is a further proof of this.
• There are legacy code or modules that are difficult to remove entirely: sometimes it is too hard to simply remove something completely if it has been there for a while, or if there are dependencies of other features on this component.
• There are different contexts for using the same features for different tasks or during different stages of the workflow: say for example, it might be more convenient to access the feature at the beginning of a task compared to the middle of a task
• They have to cater for different versions of the product and it is to try and keep some level of consistency between them: the change to ribbon design was a big jump for many users, so to allow people who are making a switch rather than completely new to the software, it creates a bridge for them to gradually move into the new UI design (or not).

There are probably other reasons, or maybe all of the reasons combined together makes it difficult to do away with redundancies (or make it necessary).

If a user can't find an option or feature, then it doesn't exist

There has to be some means by which a user who is looking for a feature can reasonably expect to find it, and by which users can browse features to learn what is available.

Well-designed menus are really good at this. Clusters of related buttons and displays too, especially with tool tips.

Keyboard shortcuts and gestures are terrible at this.

Easy things should be easy

Little is more frustrating to a user when a simple task is tedious or complicated to execute.

Navigating menus, especially hierarchical ones, is bad at this.

Buttons are pretty good. Displays that already show the information you're looking for without any action on your part are great! Easily remembered keyboard shortcuts are good too.

Often used things should be quick

This is much like the above. Optimize various interfaces so that the more frequently used things take less interaction.

Ribbons are pretty good at this if it's easy for the user to customize.

Keyboard shortcuts are essential

Good keyboard shortcuts save a vast amount of effort on the part of the user, and let them carry out tasks without breaking their flow. Triply so in applications (like a text editor!) where a user is expected to be providing keyboard input anyways.

I imagine people who aren't used to them don't really appreciate this, but for people fluent in such, a lack of keyboard access can be crippling to an interface. Even a clunky keyboard interface does terrible harm to usability.

Having them, however, pretty much requires features to appear elsewhere so they can be discovered; e.g. showing the shortcuts in menus.

It's simply very painful to remove features from established software.

Featuritus is often a marketing advantage.

The initial redundancy of being able to invoke an action via menu or keyboard is proven useful pattern - some people prefer to use the mouse (menu) and some prefer the keyboard. The menus are more discoverable but the keyboard is faster. This was the way the original Mac UI was designed and it worked well.

The toolbar (or ribbon) came later and facilitated more actions, more features, aided discoverability, and once learned was faster than menus. Even though most users might use the ribbon, some might stick with the menus because that's what they were familiar with. This amount of redundancy is generally not a good thing, but it's better than forcing people to give up what they're used to, which can be disastrous from a marketing perspective (see Windows 8).

Often this can be summed up with 2 words.

Backwards Compatibility

The original Word users likely migrated from WordPerfect.. Which was very keyboard focused cause when you type that's where your hands are.

Thus when Word first had its menus and toolbars they had to support hot keys too. When they came out with "personalized menus" where options not used frequently would disappear users revolted and forced them back to the way they were via a config setting (PS users hate change!) When they came out with "The Ribbon" to heavily mixed reactions they had yet another paradigm for users yet they tried to ensure most of the functionality under the hood was still accessible in all the ways they could.

Long story short, if you gave a user an option previously... You will struggle to remove it later (user backlash) if the users are passionate/vocal enough it can negatively affect your product positioning and brand.

Plan your UI carefully from day one... Once you get market adoption it may be quite hard to change it.

Case in point for me, I have used AutoCAD since version 9 on DOS... and have mastered "near blindfold-able" use of it via the keyboard. If Autodesk releases a version without the command line... I will never upgrade again.