What makes models unrippable or hard to rip?

[BoyOBlight121]

This is slightly related to the last thread I made where I asked if there were any unrippable models from games.

The short answer to that is "Yes, so far", but now I would like to know WHY those models are unrippable or hard to rip. Does it has something to do with the engine the game is made off of, or how the models are saved? Maybe what software the models are made on, or is it just that our technology is too limited right now?

[Demonslayerx8]

the reason why is because either..

1 - Others aren't interested in RE'ing the model format/data
2 - They have odd compression that can't be figured out and would have to be searched via memory (depends on what game) for the proper data to decompress the files
3 - Would take months to figure out even if they were interested, and would probably give up without finishing or sharing their work/progress.