MIK Sumo are not Gremlin, individuals are just that and ltd and plc companies something different again. The issue is tracking the rights holders and doing a deal that makes commercial sense, something that can be especially dificult in the case of bankruptsy (most companies in the games industry go bankrupt after one to two generations) and/or have undergone multiple take overs. Gremlin went through multiple changes, from the founding company to the US Gold take over, to a buy-back, to the infogrames years, and that itself became a mind-fuck of re-organisations and studio closures. Also, even talking about the earlier years when matters were more transparent. Gremlin was a developer/publisher, not simply a developer. That means that many games were developed by other companies, but published by and under the Gremlin label. In those cases the rights would likely reside elsewhere. Someone mentioned many rights being re-purchased and that is possible and might make it easier to track down and do a deal. But yea, former developers will almost certainly have rights, like the guys at Magnetic fields - Shaun Southern and Andrew Morris. And all this is just looking at Gremlin. In some cases it might be hard to know who if anyone owns given rights and if money becomes involved, previously unclaimed rights can become disputed.
Lucky this is not more complex again. Modern games often have licencing agreements tied to a specific system and/or further complexities with third party licences, especially sports titles. One look at Microsoft/Xbox One's lack of sports games on backwards compatibility shows off that problem. Even licences on music tracks on modern games can be problematic.
We're very lucky we can obtain most games ourselves!