Change the font on most apps and nothing structural happens - text reflows, maybe wraps differently, but the underlying content doesn't shift what it means. Typing tests are a strange exception, because the font isn't just decoration here. It's part of the measuring instrument.
In a monospace font, every character - a lowercase "i" and an uppercase "M" alike - occupies exactly the same width. In a proportional font (most fonts, including the one this sentence is probably rendered in), an "i" is narrow and an "m" is wide, so identical character counts produce visually different line lengths depending on which letters happen to be in them. That's fine for reading. It's a problem for a typing test, where your eyes are tracking a live position on a line and the highlighting engine is tracking a character index - and those two things need to agree on where "here" is at every instant.
With a proportional font, a run of narrow letters can visually compress a stretch of text tighter than your muscle memory expects, and a run of wide letters can stretch it looser. None of that changes what's correct, but it changes what the line looks like while you're mid-keystroke, which is exactly the moment you can least afford a mismatch between what your eyes see and what your fingers are doing.
That's why the default typing font is a fixed-width one - not because monospace looks more "serious," but because it's the one font category where the visual layout can't drift out of sync with the character grid the scoring engine actually uses. You're welcome to switch it - TypeClash lets you customize font, size, and color freely once you're in the seat. But the default exists because the visual instrument and the measuring instrument have to agree on the units, and monospace is the only category where that's guaranteed by construction.