When a company stops existing for whatever reason, then its assets (physical, financial, intellectual or otherwise) don't stop existing. When the company dissolves voluntarily, those assets usually go to the owner(s) of the company. When the company got bought up and integrated into another company, the buying company will usually own them. When the company went bankrupt and got liquidated, then they will often get sold off to the highest bidder.
And then there is the question of who actually owns the IP rights to a specific game asset. Often there is not just one legal person which worked on a game. In addition to the developer, there might also have been a separate publisher who might or might not own IP rights. There might have been investors in the background who financed the project and now own some copyrights. Sometimes there was more than one publisher. And sub-contractors might have been involved in the development who only licensed their assets but retained copyright.
So unfortunately it is often not really clear what happened to IP assets of a defunct company. People who might have the rights will often not care much about them until something happens which gives new commercial value to those assets. So even if you are unable to determine who owns the assets to a game, as soon as you start using them, someone might show up with a plausible claim to the IP rights and demand money from you. Such lawsuits can get really messy and really expensive.
For a good example for just how much of a goose chase you might be in for when you want to legally obtain the rights to a game from a defunct company, check out the story of the re-release of No One Lives Forever. tl;dr: multiple game companies said "Maybe we have the rights to the game according to some contract buried in some file cabinet, but we don't care enough to find out. But if you try to release it, we will find out and if we do we will sue you!"