Jango

They were a big thing about then, I remember hearing "High School Never Ends" on TV a lot. 

I've heard that for previous games Jun picked a lot of the vocalists, don't know if it happened after 06 though.

Ohtani was the sound director for both Unleashed and Forces, but his English skills are limited. He said in an interview that he usually turns to Senoue to help find artists and vocalists, and I guess that's doubly the case when it comes to vocal themes. So with that in mind, it's very likely that Senoue recommended and reached out to Jarret Reddick.

FWIW, SEGA often don't contract full bands to write or perform songs. Bowling For Soup didn't perform Endless Possibility; it was SEGA's teams on the instruments and Reddick merely sung the vocals. It's similarly the case for basically everything else. Jean Paul Maklouf of Cash Cash said it was very strange writing the lyrics for either RFTS or SWYH (I forget which) to fit a track that someone else had composed. 

They did a lot of work for Disney around the mid 2000s so I do remember them still being a pretty big deal when Unleashed started making its rounds. Though, I do think this was at the tail end of that popularity. From what I remember, their genre in general peaked before the 2010s.

Makes sense now when thinking back how most songs would credit one or two vocalists from their respective bands/groups in the descriptions rather than crediting the whole band/group, except for the few exceptions like the Zebrahead version of His World where the whole group was there.