We don't have the ability to preserve all information for all time. Storage media decays, formats obsolete, storage imposes cost, space and time considerations that are non-zero and non-trivial.
If these sites were a special instance of our culture, then fine. But they're not. We don't need every single episode of Eastenders archived for all time to understand culture. A good sample is way better.
These are different times to the classical era. We have a glut of information, most of which, once archived, will never be looked at again, and will only cost money until finally abandoned.
I went to a museum once where they just kept everything, I learnt 2 things, one, that the first radio batteries came in more varieties than we have today, and two, never go to a museum that try's to keep everything...
If these sites were a special instance of our culture, then fine. But they're not. We don't need every single episode of Eastenders archived for all time to understand culture. A good sample is way better.
These are different times to the classical era. We have a glut of information, most of which, once archived, will never be looked at again, and will only cost money until finally abandoned.