I was just on cssfontstack.
It says 98.74% of Macs have the font arial.
Does that mean that over 1 in 100 Mac users are intentionally uninstalling arial and if so why?
The statistics on that site are not reliable. It lists Impact as 0% on Windows, even though it's been preinstalled on Windows since Windows 98 and Internet Explorer 3. It lists Rockwell Extra bold as 66.1% on Windows and 0% on macOS, even though to the best of my knowledge it comes with Microsoft Office on both OSes. I do not know where the statistics come from, how they are collected, processed or anything like that. The only thing that's for certain is that whatever numbers it shows there can at best be used as a guideline, not as a definitive truth down to two significant digits.
Additionally, the site only is concerned about Windows and macOS, leaving out >60% of all users (who are mostly on Android or iOS).
When specifying a font, make sure you always provide fallback fonts, and also, test your app on several devices to see how it looks
But to answer the question arising out of these questionable stats:
Do people uninstall standard fonts?
Generally not, no.