When it costs you to build a board let's say 10 000 USD, would you trust libraries downloaded from somewhere? That is one of the reasons why professional companies always create their own libraries - you need to have 100% confidence in them, otherwise if there is for example a wrong footprint it can cost you a lot of money and a lot of time to fix it.
Another big reason is, everyone is following different standards - e.g. what layers are used, how they draw outlines or mark pin 1, how they rotate components in libraries .... etc. In your own library, you would like to have it consistent - if you keep downloading libraries from different places, your library will not be consistent or you have to update the footprint / symbol anyway.
And you have to always check the downloaded symbols and footprints. So, often it is just faster to create the symbol and footprint by yourself. Sometimes it may help if you download it (it can save some time from manually drawing it), but you still will need to do some work with it.
Many people tried to "standardize" libraries, but I still have not seen one single place which would be used by everyone.
PS: Yes, there is IPC standard, it helps, but there are still many footprints where even IPC generated footprints are adjusted.
- from my experience the libraries between altium versions are usually compatible
- the closest I have seen to standards is probably PCB Libraries https://www.pcblibraries.com/