If you know how to select Comic Sans, you know how to select a different serif like Times. In default Windows installs there's at least 3-5 other choices for "serious serif font". And people do that, also the less tech-savvy ones: if they just want to type their homework, it's Times (or Arial), no second thought. But if it's something they want to "stand out" or be different they can easily pick a substitute like Georgia from the list.
But if people just write a note, or anything they intend to come across as "not too stern, not too serious, but also not too boring" there is just Comic Sans, in the default list of Windows fonts.
Like another commenter said, it's like a market is underserved. Except it's not a market, because it's Windows' default font list (there's enough alternatives on the market). It needs another informal, slightly quirky font if you want to break the reign of Comic Sans. They might have gone a bit in this direction with Calibri (subtly rounded) and Candara (playful) but those are way too subtle to notice in the (oddly tiny) font selection lists.
And something else, IMO, Times News Roman is for me the example that, apart from popularity and being an over-used default font, there is something extra wrong with Comic Sans. After all those years of being negatively conditioned with Times as the default Apache 404 Error font, and so many other places, all that's caused is a strong notion of neutrality and blandness. I don't dislike Times, and I also believe there is no discussion on that it's strictly better designed than Comic Sans: it's got more glyphs, the letter forms are more constant and harmonious, etc. And with some thought[0], you can combine it with another font (Arial being the obvious choice) and use it in a headline/body text combo, and it actually doesn't even look half bad. Somebody put some thought in that, it looks clean and they just happen to be two of the most-used fonts. I challenge anyone to pull that off with Comic Sans, even in combination with another font.
Personally I think, add just one cool informal font to the Windows default list, and IMO it will be fine. Really the thing that bothers me most about Comic Sans is that it's the only choice [for people not savvy enough to install extra fonts], so I can't really fault anyone for using it because any alternative I could suggest involves quite a bit of extra work for them (in addition it might revert to some default font on the next office PC where it's not available).
[0] this almost always means: whitespace