Blocking individual accounts is something Parler can do (and chose not to), not Amazon/Facebook/Google who don't have access to the contents of those accounts (at least not easily).

Also, it's worth looking at some of the hacked info that was released - Parler had a serious moderation system, but it was used to make sure people had MAGA viewpoints, not to prevent violence. This is not some free and open platform, it was a controlled one that purposely built an echo chamber of violent rhetoric. Twitter and Facebook don't allow the same kind of violent rhetoric, lies, etc., and while they may not do an ideal (or even good) job at it, the fact is they're broad platforms used for a whole lot of things for a whole lot of people and are absolutely not comparable to Parler.