Not all libraries* include sample packs* but most downloads do come with either wav or aif files.
I can think of a couple of reasons why it happens;
Some library creators may not wish to share the samples but don’t mind offering a finished instrument.
Some may have baulked at uploading an extra couple of gigabits on top of the finished item.
I am assuming what you would like, is to be able to see immediately which libraries have packs of aif or wav in the files, which only have embedded sound files and which are in monolith format and so on. That would certainly help all the people who are coming just for ‘Sample Packs*’.
I have that info to hand but even if I get the time to share it, it will not contain direct links, so you will have to browse the site and have my list to refer to. And it will take me a couple of weeks to get out.
*Before 12th October 2021 Pianobook hosted libraries so the libraries rather than the samples were what most people were looking for.
*From 12th October 2021 Pianobook started calling the libraries ‘Sample Packs’ and this has increased the number of visitors to the forum, looking for the Sample Packs.