I understand that the suffix
.cls stands for classes.
Then, why packages don't have the suffix
.pkg, but rather have
LaTeX 2.09 did not have the distinction; everything was called
When we made LaTeX2e, there were relatively few "main document styles". The standard ones (article/report/book) accounted for the vast majority of documents. As they were part of the core distribution, changing those to be "classes" was not too hard.
The majority of "style files" worked unchanged or with only minor updates with 2e, so forcing a filename change would have been too disruptive, but "style" seemed to be the wrong name, so "package" was introduced.
This is an historical artifact.
In LaTeX 2.09 there weren't "document classes", but "document styles", with extension
Additional "packages" were loaded as options to
When LaTeX2e introduced document classes (extension