Why are primarysources that are not yet on the internet not used, for example probates?
If you are referring to using sources that require more than an internet URL for supporting evidence in Family Tree, there is no reason not to use any primary source that is available. A URL is not required for a source citation. If copyright and such allow, you can scan or photograph a document and upload it to use as a source. At a minimum, you can just enter a bibliographic citation as the source. You can enter information from a source in the notes section.
If primary sources are not on the internet, then access to the paper version will be a problem. Most people won't be able to use them because most people won't be able to travel to see them. Those who can physically go somewhere to look at a record can take a photo with their phone and upload it will be in the minority. If you can upload such a photo to memories, then you can create a source from it.
By the way, probate records is not a good example. There are literally billions of them online. The state of Virginia alone has 1,378,247 records in Ancestry classified as wills and probate. But I get your point.