Not all games are watched live, if something catches the eye of a surveillance officer they may watch the table live. There are cameras recording all tables and most of the time surveillance is called to review the video (notice I did not say tape, most major casinos are digital nowadays). There just is not enough officers to watch all the games live, there are other things going on in a casino, count, table drop, slot drop, escorts, security incidents, verification of payouts, reviewing variances, on and on, they have a lot to do, plus they may have assigned duties like reviewing video for investigations during their shift.
Not saying they don't watch tables live, they will watch tables for P&P violations by dealers and supervisors and issue reports for discipline. It's a busy world in surveillance. I have sat in surveillance enough reviewing video over the years, they usually have a small crew and they keep busy where I work.

Also... Roulette chips walk off the tables all the time - especially with novice gamblers, they are always trying to redeem them at the cage.