Because Mike works for Gus Fring, presumably he should be all set financially. Also Mike is apparently loyal to Gus. Why would he need a part time job as a PI for Saul Goodman on top of his multiple activities for Gu's operation? This part-time job might even be a conflict of interests. Also Mike is obviously old and not very fit - why does he want extra work?
Why would Gus allow his loyal hit man do part time work on the side?