Actually... they do. And they do a lot, honestly.
Jiena was a finalist in the avex Voice Actor Auditions last summer; she made it to the final four. The auditions were based in showroom, and she decided to continue doing them because she enjoyed it. She also takes lessons in voice acting!
Hitomi performed in a stage play in February, and has done modeling for Bandai and other magazines. I don't know which ones, but I do know she has shared some photos from both fashion and hair modeling. She also appeared in Kamen Rider Drive with a returning role (that even returned for the V cinema Kamen Rider Chaser).
Chisato had a solo live event in January, performed in last year's Sengoku Basara anniversary AND recorded a song for the CD, put out a gravure DVD, and has also done Bandai modeling. She takes voice acting lessons with Jiena, and got her first voice acting part in the Kamen Rider Mach drama CD.
Aya just finished her second run in the stage play Montage, has done several others, assisted on a variety show, has done a tv drama, had a gravure photoshoot with her sister Amaki Jun and constantly takes dance lessons. She choreographed one of the KRGS B sides for live performance last winter (Scarlet Savage).
And all of them assist on Radiranger. They just take turns. It's unusual for more than two of them to appear at a time.