Given the assets between Sony and Fox, Sony's key assets don't seem very valuable from the entire business standpoint of view if Comcast goes through buying Sony Entertainment. Also, MGM doesn't have a lot of interesting IPs (aside from James Bond, which is a foreign property rather than American) and isn't interested in selling itself to Comcast, so it just makes the purchase of Sony's key assets look terrible from a business and financial strategic point of view, and further purchases of other companies would take a long more time and scrutiny with anti-trust regulators and shareholders who have different views of how the operation should proceed. Disney bought Marvel in order to appeal to the boys and men and to sell a bunch of toys, Disney bought Lucasfilm because of its long-term relationship with former creative consultant George Lucas and it already has a Star Wars attraction in its parks, and Comcast bought DreamWorks Animation because of its relations (since DWA is headquartered in the same backlot as Universal Studios) with the company and to expand its animation empire to catch up with Disney. All three assets provide strategic value to both companies, with the House of Mouse growing into a powerhouse thanks to the strong popularity of Marvel and Lucasfilm.
Sony has dozen of popular IPs like Spider-Man, Jump Street, MiB, and Ghostbusters, but doesn't own any popular cable networks (e.g., Disney's Freeform, Fox's FX, Time Warner's TNT, Paramount's Comedy Central, and Comcast's USA Network), doesn't own a sports network, doesn't have a news network, and doesn't own a percentage of Hulu. From Comcast's perspective, even if they want to expand its presence in Asia, Sony's Hollywood side won't benefit them in the long run, except losing tons of money from this their future acquisitions would be affected. Not worth the risk.
Comcast's only chance is that should the AT&T-TW merger be approved, they might gun to outbid Disney for Fox, as it would provide strategic value to the company, like Marvel and Lucasfilm have for Disney.