Sandboxing does use some RAM, but it was a big win for security. One site can’t crash the entire browser or use a security hole to get access to data on other tabs. Still, the majority of the RAM is taken by the site itself. The processes do share some RAM - they’re not entirely isolated.
No, the reason why browsers use so much RAM is because every tab is it’s own process and sandbox. That and lazy handling of content.
Sandboxing does use some RAM, but it was a big win for security. One site can’t crash the entire browser or use a security hole to get access to data on other tabs. Still, the majority of the RAM is taken by the site itself. The processes do share some RAM - they’re not entirely isolated.