We are used to finding everything on computers and expect most things to be available online. Many archival collections were catalogued before the internet became so widely used. This means that many catalogues only exist on paper or as index cards.
Many repositories are attempting to retro-convert such catalogues (i.e. convert old catalogues to digital to make the information searchable on computers) but this is very time-consuming. This is one reason that archives often provide the fonds level description online, without the lower level descriptions.
This is particularly common in resources such as the Archives Hub (see section 3). This means that it can be worth asking the archivists at the relevant repository whether a more detailed catalogue is available, although you still can’t expect every single item in a file to be listed individually in all cases.