I know of at least one record label that specialises in releasing game music and I’ve seen Amon Tobin (producer who make the soundtrack for a Splinter Cell game, amongst other things) live.
Movie and game composers are literally work for hire. It's their employers who may or may not release a record related to that employer's work that may or may not credit the people involved.
Even the extremely successful and popular composers are not necessarily releasing records or doing live shows. Even John Williams is primarily a conductor and a classical composer who didn't really start "touring" until 2002 or so. Same for Hans Zimmer. He doesn't release "records". Studios hiring him release movie soundtracks for which he was specifically hired. Etc.
According to your definition of "legitimate artists" these artists are not legitimate.