I understand why that seems like an appealing solution, but there are some pitfalls with creating multiple distinct UIs:
The most obvious pitfall is that more UIs = more work. There's the initial development, but it also increases the work every time you need to add something to the UI, and increases the likelihood of bugs. Microsoft could afford this increased cost, but it's not clear if it would be worth it.
Secondly, having multiple distinct UIs makes it difficult for someone to transition from a "casual user" to a "power user". If someone who has only used the "simple" UI switches to the "power" UI, they have to re-learn everything from scratch. Additionally, if there are any features limited to the "power" UI, it's extremely difficult for a "simple" UI user to discover those features even exist.
That doesn't mean that creating multiple UIs isn't ultimately the right solution for MS Office. It might be! But doing that comes with downsides, and I can understand why Microsoft doesn't want to go in that direction.
The most obvious pitfall is that more UIs = more work. There's the initial development, but it also increases the work every time you need to add something to the UI, and increases the likelihood of bugs. Microsoft could afford this increased cost, but it's not clear if it would be worth it.
Secondly, having multiple distinct UIs makes it difficult for someone to transition from a "casual user" to a "power user". If someone who has only used the "simple" UI switches to the "power" UI, they have to re-learn everything from scratch. Additionally, if there are any features limited to the "power" UI, it's extremely difficult for a "simple" UI user to discover those features even exist.
That doesn't mean that creating multiple UIs isn't ultimately the right solution for MS Office. It might be! But doing that comes with downsides, and I can understand why Microsoft doesn't want to go in that direction.