The answer, pure and simple -- and as it usually is in Las Vegas -- is that they generate a lot of money.
In 2012, Forbes magazine began publishing its now-annual "Electronic Cash Kings" list, detailing the highest-paid DJs in the world. At that time, Calvin Harris was the top earner: "Born Adam Richard Wiles, the former grocery-store stocker earned $66 million in our scoring period, playing more than 125 gigs. He's found great success in the pop world, working with the likes of Rihanna and Kesha, which has helped him land headline gigs not only at EDM-focused events, but also at festivals like Coachella. Harris continues to play regularly in Las Vegas where he has a multi-year deal at America's biggest nightclub, Hakkasan."
Harris has been the top earner every year since then, though his 2016 earnings were down to $63 million (poor guy). Tiësto was second in 2016, with a mere $38 million. French DJ David Guetta ranked third with $28 million.
Harris' little biography above points to one thing about today's DJs that sets them apart from the vinyl spinners of yesteryear: Disc jockeys traditionally introduced songs to the public by playing them on the radio or in discos, and while they had a significant role in popularizing tunes and artists, they weren't tunesters or artists in their own right, nor were they producers or collaborators on the tracks they spun.
The EDM era, however, ushered in the trend both for remixes of already popular songs and for DJ creators in their own right -- Calvin Harris, who hails from Scotland, saved up from that supermarket job and from working in a fish-processing factory to buy the equipment that launched his career from his MySpace page. Having been "discovered," in 2006 he signed publishing and recording deals with EMI and Sony BMG, respectively, and the following year released his debut album, "I Created Disco," from which the first single released was titled "Vegas," appropriately enough.
The same holds true for other big-name DJs like Dutch-born Tiësto, who has a number of studio albums to his name, in addition to a bunch of remixes; he's also a regular at Hakkasan.
French-born David Guetta plays a couple doze gigs a year in Las Vegas, while also holding down a weekly residency on the European island of Ibiza (among other things).
While the figures they earn may seem ludicrous to the layman, today's megastar DJs are showmen who entertain festival and nightclub crowds that were undreamed of back in the day, while often undertaking grueling travel schedules.