Luthor is supposed to be appealing and look like a really good guy at face value, a guy you would always vote for, a person you would really like, a charismatic and fantastic speaker, a person who you could see becoming The President one day, whilst behind closed doors is an intimidating and ruthless man with no moral conscience who has a presence and swag.
Lex isn't some nerdy, snarky techno geek loner Zuckerberg type
Luthor is this
Can you really see Michael Cera telling someone like The Joker what to do with it coming across as convincing and threatening, fukk no