Why running?
To be VP on a Trump ticket, be in the cabinet, get his name out there as an investment in a political future. Probably a few other reasons.
Until the anointing at the convention it’s still possible Trump could fall. That would be an opportunity for him. He could credibly argue that he has picked up the MAGA cause since he’s been openly Trump supportive.
I would like Trump to choose him as his VP. Then I’d like Trump to treat VR as he would a vice president in his business organization, that is, give him some real responsibilities and authority.