Why would anyone use anything other than brain wallet?
Because people are bad at "random". Plenty of brain wallets have been "hacked". I have a hard time calling it a hack honestly, its more like guessing your phrase and boom your bitcoin belong to me now.
I mean installed on your own computer, not running on someone else's server.
installed? I am not sure you understand how a brainwallet works.
Is there some reason why it is not the most secure, easiest to use wallet on the planet?
Please explain what exactly you understand a "brainwallet" as and Ill gladly point out its flaws.