Historian here.
What's on those sites is based off source materials. And those are generally disparate: oral witnesses, family members, documents found in private archives,... but also public records found in your city archives.
Where I live, different offices kept population records through out time e.g. city hall, the Church,... You can easily walk in, and access records from the 18th century and figure out when someone was born, who they married to and when they died.
However, legal provisions prohibit access to those public records pertaining to births, marriages and deaths which are less then 100 years old. Meaning you don't get access to lists of names of all everyone were born in 1936.
Why? Privacy for the living. e.g. someone could walk in and assert that your parents aren't your real parents.
Moreover, privacy legislation states that you need consent from living people before you can publish personal information regarding their names, addresses and so on. It means that even if someone passes away, you can't just publish that they were married or about their children if any of those are still alive.
Publishing pictures is even more fraught. You need consent from the person depicted, and consent from the photographer. That means ascertaining whether they are still alive, or getting consent from their estate in the case of the photographer. In the latter case, it might mean you may have to wait for 70 years after their death before those pictures enter the public domain. Genealogy websites allowing people to simply upload pictures without constraints expose themselves to copyright liability in that regard.
... and then there's the issue of how those legal provisions are upheld on the Internet. For instance, any ancestry/genealogy site who wants to operate in the EU has to comply with EU legislation including GDPR, copyright, right to be forgotten and so on.
For sure, being able to dig into your own ancestry and being able to rely on other people's work through digital technology is awesome. But, as with anything digital, it does come with the same challenges that confront any business case leveraging personal information.
TL;DR: Genealogy is fraught with legal challenges. You don't just get to publish, let alone scour and use those data without consent.