I feel like this is the same thing as... "My neighbor holds a party at night just because the cockroach in their house got their birthdays." And doesn't let us sleep due to their endless karaoke streak.
Ritsuka is a mecha otaku... or at least in this comic.
They command an army... A moving fortress is perfect fit, no?
There are servants who are willing to build a Gundam or futuristic fortress for them XD
I'm pretty sure Ritsuka also like them flashy. Most rider's NP are like that.