While real life PIs could find themselves getting into some hairy situations (Allen Pinkerton got famous for stopping an assassination attempt on Abraham Lincoln), most movies overlook certain aspects of their lives. Either that or the editors were on the ball when they decided to weed out the countless hours of sitting around doing just short of jack shit.
Many private investigators work for insurance companies or collection agencies, tracking down deadbeats and people trying to scam their way into a disability claim. So you're basically spending hours and hours trying to track down an address, or following some fat guy around who claims his back is too injured to work, waiting for him to pick up something heavy so you can snap a picture of it.
There's also the adultery cases, wives wanting to catch husbands in the act to force a better divorce settlement. This means following the same fat guy around town while he goes about his tedious routine, with the goal of getting photos of him having fat sex with some lady who isn't his wife, praying the whole time that this does not in fact happen.
In reality very rarely do private investigators investigate or solve crimes, since, you know, that's what the cops are for.
Investigators starting out in the industry with an agency can expect to earn around $30,000 a year and will likely only resemble Magnum PI if they elect to grow a giant mustache. The likelihood of a rich benefactor wandering in to ask a PI to find her husband's killer and handing over an envelope full of cash is about as likely as Jessica Biel walking in and asking for a boob massage.