This is a common a misconception or well at least not true for everyone kind of thing.
I've bought a mouse with such holes in 2022 january so like 2 and half years ago and I'm yet to take apart the mouse to clean it out and this is how it looks like right now:
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What you can see inside is basically simply dust build up
'we also had some renovation work done recently and I did not cover the mouse' but no gunk anywhere and to be honest I've had nastier looking full cover/solid mouses in the past that just kept collecting all kind of crap for whatever reason.
That being said, I am a fairly clean person and I don't eat greasy food at my PC and I try to keep my hands clean all the time and the same goes for my room in general.
I will take apart the mouse soon tho since I want to clean the dust out, I do the same to my keyboard every now and then.
Edit:
Took me ~15 mins, good as new and this much of a maintance I do to every mouse anyway.
'I do clean the outside of the mouse more often ofc'
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Edit 2:
Now that I think about it I think my full cover/solid mouses were nastier cause my hands used to sweat easier on those but with the hole mouse it doesn't happen at all so ye I will probably never go back to those.