There isn't one, really. Because that's what makes them identifiable as Kamen Riders to the public. There's no other reason, save it's the best for the suit actor. I'd argue that neither Gaim or Ex-Aid have bug eyes in the traditional Rider style (Ex-Aid is obviously cribbing SD Gundam). Ryuki's supposed to be a dragon.

Probably because of traditional. Though you're slightly wrong on a few things
Kuuga: Bug, Stag Beetle
Agito: Bug, Stag Beetle
Ryuki: Not a bug, but you'd see the knight-like thing instead
Faiz: Not a bug, but stylized helmet to resemble, well, the Phi symbol
Blade: Bug, Scarab Beetle
Hibiki: No Bug eyes
Kabuto: Bug, Rhinoceros Beetle
Den-O: Not a bug, but the eyes are shaped like a peach, which is another reference to Momotarou, Rod form has bug eyes, but that's more likely because of the turtle motive as some turtles have spots on their shells iirc
Kiva: Not a bug, but stylized like a Jack-o'-lantern
Decade: Not a bug, but he's supposed to be the anniversary thing
W: Bug, and W as a whole is a shout out to Showa
OOO: Occasionally a bug, but they likely kept the bug eyes to save on helmet designs
Fourze: You win
Wizard: No bug eyes
Gaim: Not a bug, It's stylized like samurai armour
Drive: Not a bug, stylized as a car
Ghost: Bug Firefly, but no bug eyes
Ex-Aid: No bug eyes

Decade's suit was different, the actor would have to look through the barcode instead of the eyes iirc, they showed it in a net movie

Agito's actually a dragon, not a beetle.

Pretty much right about the rest though.

His horns before using a finisher always reminded me of stag beetles, my bad

Agito's horns always made me think of a frilled lizard. I have a hard time seeing the dragon.

WOW. That's nuts! Is Decade's design a barcode? I thought they were supposed to be polaroids to go along with the camera belt and history motifffff. ;}
I still haven't finished Decade; it's not terribly compelling.

"Kao ga barcode"
Decade Eyecon. So yeah. And Decade is trash anyway

I thought we just established he's a barcode? :p

sunnova-
Well played Fishy Friendo!