That might be your experience but it's not mine. If a negative is encased in a protective sleeve, then there is no way dust could get onto it but maybe they were exposed to dust as you removed them from the sleeve?
The sleeves were not sealed but were open at one end just as they came from the developer. Maybe over decades dust worked its way in that way. Some were worse than others.
I usually used 36 exposure 35mm films. Of course, the 2 1/4 was either 8 or 12 per roll. I used individual sized envelopes for the 4 x 5 negatives.
I placed negatives from each roll in a Legal size envelope and labeled the content and date for the exposures. The enveloped were placed in a box (on edge - NOT flat) so this kept the media normal and without possibility of getting any dust or creases.
In later years, I have scanned all Family and Friends oriented negatives and slides. The proper care for the media really "paid off" with basically NO problems regarding dust or other contamination. This was a rather long and devoted project since this included thousands of images. I did not scan the NON family images such as were exposed relating to commercial (business) projects for several years.