I think because there's no career advancement at Microsoft, working on cleaning up existing features. Only by working on new stuff can you get noticed.
So, interns work on 'old' stuff (like office apps, Windows, tools). And they always look pretty rough.
Interns don’t do maintenance work in general. They build new stuff, often flashy new stuff that never ships [1], because the intern program is about evaluating potential hires and giving them a great experience so they want to come back. Giving interns tedious unrecognized maintenance work is a great way to convince them to go elsewhere.
Source: Microsoft employee
[1] The goal is still to build features that ship, but because they get new, shiny stuff, sometimes it’s speculative development and the winds shift before it goes out the door.
That's pretty much how it works everywhere. There's no career, or even pay, advancement for people who like to do this sort of work. You're called a "maintenance programmer" which people take to mean you're not very good. On the contrary, a lot of those types of developers are very good. Better than the people who initially wrote the code and jumped ship when the bugs started rolling in that they couldn't fix. They understand the system and the business much better than the talky-talky system architecture wanna-bes.
Whew, that turned into a rant quickly. I've just seen plenty of strong, smart developers get screwed because they don't do flashy work.
So, interns work on 'old' stuff (like office apps, Windows, tools). And they always look pretty rough.