Emulators and game engine clones may encourage "stealing", but they are also unique creations. The people who develop said software are typically careful about keeping their software separate from copyrighted materials. In the case of OpenTTD, they did so by creating their own graphics and sound assets to accompany the game engine.

If you are claiming that creating an independent clone of the game engine is stealing, you are entitled to your own opinion. But do understand that it is an opinion and not a fact.

I would also ask you to consider the consequences if that opinion were codified into law. It would make all forms of progress (e.g. literary and technical) nearly impossible since nearly all ideas are derivative. To give an example: the computing landscape would be very different. IBM compatibles would not have been a thing, leaving the market either fragmented or consolidated in the hands of a single company. Oh sure, there were companies that did steal by producing verbatim copies of the IBM PC ROM or the mainboard layout ... but we are talking about a reimplementation in the case of IBM compatibles and OpenTTD, not copies.

Come to think of it, the entire computer industry would have been set back by decades with an excessively strong IP regime. No one seriously classifies the ABC as the first computer, yet the courts used it to strike down patents on early computers. In the early days, IBM played games with IP licensing to try to restrict their competition, something the courts shot down. AT&T didn't give away Unix, nor did they license patents on transistors out of the goodness of their heart. They did so because regulators and the courts recognized that IP could be used to stifle competition (and, by extension, it would have inhibited progress). So I doubt that the courts would agree on emulation or game engine clones being stealing either.