One reason is to make a FOSS version, and other reason can be to add improvements. Furthermore, with some game engines, you might then be able to make your own

One reason is to make a FOSS version, and other reason can be to add improvements. Furthermore, with some game engines, you might then be able to make your own sets of levels and other stuff, without needing the original proprietary software to be used with. You might also try to figure out the behaviour of specific details of the game engine, so that the FOSS version of the program may explain it. You might also be able to port it to other computers. And, maybe, you just like to do it. This can be the case with many other game engines too. In a few cases, the original code may later be released as FOSS anyways (or, in the case of ZZT, it was rewritten from disassembling the code and made to produce the same executable file), but sometimes it might not. (I had made a FOSS implementation of Everett Kaser's MESH:Hero game engine, and at least, the reasons I mentioned above are my own reasons for doing so; I don't know if the people who did TR1X have similar ideas and/or different ideas.)