Marc Levin, Flickr. 

Among other interesting tidbits, Forbes' latest list of the world's highest-paid athletes highlights the idea that golfers, more so than other athletes, are sponsorship goldmines. Tiger Woods' $55 million in endorsement income last year was second to none, and Phil Mickelson nearly matched that, raking in $48 million.

Both pros are more sought after than once-in-a-generation superstars like Kobe Bryant, Cristiano Ronaldo, and Peyton Manning. And on average, the PGA Tour's three biggest stars -- Woods, Mickelson, and Rory McIlroy -- made more money from sponsors in 2013 than the top trios in the NFL, MLB, NBA, ATP World Tour, and any of the international football leagues. The obvious question is: Why are golfers so marketable?

It starts with the demographics
According to a Harris Poll, men's golf ranks a mere 11th in the country in terms of fandom, behind all four major North American sports and so-called "fringe sports" like boxing and tennis.

What the PGA Tour lacks in mainstream viewers, however, it makes up for in geezers. Nielsen reports that golf has the oldest fan base when compared to its counterparts -- 63% of fans are above the age of 55, a split nearly twice as large as most other sports. And not surprisingly, with age comes affluence. Almost a quarter of all golf fans generate at least $100,000 a year in income.

Nielsen also reports that, despite their seniority, this group is tech-friendly. Americans over 55 typically watch more TV than their peers, and spend more time online than children and young adults between the ages of 12 and 24. Golf fans, in particular, prefer to track the PGA Tour via smartphone and tablet more often than followers of other sports do, the outlet says.   

Digging deeper
Access to a unique fan base -- with purchasing power -- is a main reason sponsors pay golfers the big bucks. But is there something else going on? I recently chatted with Jim Andrews, a sponsorship expert with IEG, to find out.

"For many but not all brands, golfers have long been at the top of the list for endorsement deals because they have a number of factors in their favor," he says. These include an "upscale demographic; generally long-term careers with many years in the public spotlight; and participation in an individual—rather than a team—sport, which allows their individual accomplishments and personalities to stand out."

The latter point is key, and is a huge reason that Woods and Mickelson make tens of millions in endorsements each year. Both have won at least five major championships, and including McIlroy, the trio is often in contention on Sundays. Golf's ability to feature individual accomplishments also explains why rising stars like Jordan Spieth and Rickie Fowler found sponsors so quickly after initial success on Tour.

Fan interaction is important too, Andrews says. Because of its focus on individuality, viewers can delve deeper into golfers' personalities than team sports allow. Look no further than the AT&T (T -1.57%)-sponsored Pebble Beach National Pro-Am, and exhibition events like Woods and McIlroy's 'Duel at Jinsha Lake' in 2012 to see this strategy in play. 

The bottom line
AT&T, Rolex, NetJets, and apparel behemoths Under Armour (UAA -1.83%) and Nike (NKE 0.14%) are just a few of golf's biggest brands. The latter maintains multi-million dollar relationships with Woods and McIlroy, while Under Armour signed a major deal with Spieth last year.

Interest from these sponsors, and others, is surprising if success is gauged solely by viewership. Fortunately for golf, the sport has many other things going for it. And fortunately for its players, their wallets are ballooning as a result.