You'd be hesitant to trust a brand if it can't keep consistent styling. Branding helps users identify a brand and believe it or not the aesthetics of a brand make a great deal of impact on consumers.

As others have said, your point comes across as "let's remove design who cares" because design and human computer interaction roles stopped where your understanding ends. Everything looks the same to you after all (it doesn't, you just haven't noticed it affecting your decision making).

It’s subtle, but attention to detail all around will add up to something that looks polished. I appreciate that as a user, at least

Fine, I’ll take the bait. If this is true, then why isn’t everything in the world Arial/Helvetica?

The designers are generally the ones doing and watching presentations on design. They are also the users of the office suite in this case.