Buying game assets is simply buying resources to use in your game, instead of making them yourself. It saves times and adds a skillset a developer might not have.
Tutorials and full games: Shows people how to write a game which is ideal when they're starting out and don't know how to work out certain game mechanics themselves. The idea is for them to develop their own game based on the tutorial. Here if your game is not significantly different from the tutorial or game demo, you'd run into trouble. For example, Google Play Store updated their terms to state only the original creator can publish a game demo. I believe it's because people were simply republishing it without changing anything which resulted in many clones of the same game.
You can also find games that developers started but didn't finish, and they sell the source code as is. In my experience, troubleshooting a game can take longer than actually writing the game. I'd be wary of those. If the game was complete and polished, they'd publish it, not sell the source code. They could also be selling the source code because the project became too big for them or they lost interest. Depending on how well their game is documented, it might be simpler to write a game from scratch rather than try to understand and amend their code.
A successful game development studio would either require a lot of time, or money. Few developers are lucky because their games capture the imagination. For most of us, if we don't have a large network (say like college students would have), or social media followers we need a lot of money to promote our games and get them noticed.
It's not something you'd just jump into, unless you had the money to pay a team of people to work in the studio - like graphic artists, programmers etc. But the more people you add to the project, the less likely it is you'll make a profit from the project. If you're new or not paying, you're also more likely to attract developers who are new. Inexperience can delay a project. I still make mistakes when I write games, but the difference is that now I can immediately spot them and know what's wrong - like I forgot to put in a safeguard there. Previously, it would take me days to try and figure out why something wasn't working right. That's the difference experience makes.
Publishers do buy the rights to games etc. but they tend to buy distribution rights. It's still up to the developer to support the game. There are many new publishers trying to get a foot into the market, but they've only had one successful title and game developers are wary of them - in other words, that game might have been successful with or without that publisher. One can't readily see what value the publisher added. This is especially true if you track the publisher's social media and they have few followers.