Why goes to motive, and there's generally more than one motive. I still think he plans to make a lot of money off the project, through subsidiaries. That's ONE of his motives, but he also at least partially believes his own lies about National Security (Which he's a total expert on) Crime, Immigration, and making American great (For white people) again.
So, let's just assume one of those is His primary motive. Does that Trump the other 5? Honestly, why anyone in particular thinks building a wall across our southern border is a good idea, that's pretty moot compared to how Impossible it is, and how much it's going to bankrupt our already crippled economy, how much it's progressively wrecking our international relations, and how many (Nonwhite) people's lives it's destroying.
I honestly care more about the people it's destroying than the insane reasons behind it. GIGO.