The landscape has changed in 25 years. Today most of your major Vegas casinos have locations across the country. Many of them have built 'racebooks' that focus on horse racing but can easily be made into a sportsbook.
After living in Vegas for nearly 2 years, I can say "major Vegas Casinos" as a term is a bit of a misnomer. There are a handful of major corporations that operate casinos in Las Vegas. They include MGM, Caesars, Station, Wynn, Boyd, etc. And a bunch of more independent casinos (operating downtown LV and on the Strip), including Virgin (recently purchased the Hard Rock Hotel), SLS, The Strat (recently renamed from the Stratosphere), TI (Treasure Island), Polo, Cosmopolitan, Aria, etc.
Boyd: Aliante (NLV), Orleans (NS), Gold Coast (NS), Sun Coast, Sam's Town, Eastside Cannery, Cannery (NLV), California (DT), Fremont (DT), Main Street (DT)
Caesars: Caesars Palace (ST), Cromwell (ST), Linq (ST), Planet Hollywood (ST), Bally's (DT), Rio (NS), Paris (ST), Harrah's (ST), Nobu (ST), Flamingo (ST)
MGM: T-Mobile Arena (ST), Park Theatre (ST), Aria (ST), Bellagio (ST), Circus Circus (ST), Delano (ST), Excalibur (ST), Luxor (ST), Mandalay Bay (ST), MGM: Grand (ST), Mirage (ST), Park MGM (ST; formerly Monte Carlo), New York-New York (ST), Signature at MGM Grand (NS), Vdara (ST)
Sands: Palazzo (ST), Venetian (ST)
Station: Boulder, Fiesta Henderson, Fiesta Rancho, Green Valley Ranch, Palace (NS), Palms (NS), Red Rock (Summerlin), Santa Fe (NW), Sunset (H), Texas (NW)
Wynn: Encore (ST), Wynn (ST)
Some of these organizations have operations elsewhere in the US and/or internationally.
Wonder how legalization of sports gambling would affect Vegas economy.
Doesn't seem to be impacting Las Vegas that much, if at all. LV has been marketing itself as the entertainment capital, less emphasis on gambling, more family friendly (mayor and city are also trying to get into medical stuff, not only cutting edge technology, but also a possible place where folks can come for their medical procedures). Lots of convention business. EDC was this past weekend and had hundreds of thousands (150k+) in attendance. Many major musical groups either come for a residency and/or make LV a tour stop.