That’s like asking why the KKK don’t just do their own, WASP-only community that doesn’t interact with the rest of the world: it’s about power and “putting other people into their place”

 

Yep... also stupid. I respect the amish for their peaceful separation!

 

I would think that the purebloods sit on boards and "know people" who control industries like:

• Newspapers
• Hospitals
• Recreation areas
• Educational opportunities
• Police
• regulated monopolies on certain industries

They pass control of these things amongst each other by an unwritten "code". Control of institutions brings control of resources and people. They might also own certain magical items or spells which have been in the family for generations and grant them additional control.

Politicians today bargain over what money gets spent in laws and who has access to socially prestigious events. Much the same there I suppose.

 

They maintain control the same way the ultra elite have always controlled society.

They own the means of production, the media outlets, and politicians. This lets them control the economy, public perception, and the law. This is how it's always been done, this is how it is currently done, and this is how it happens in the HP Universe.