It's worth mentioning that slimes are fantasy versions of real world organisms:
Slime mold - Wikipedia so they can be perfectly natural organisms.
It a frequent trope that slimes are created by
magical accident. An experiment goes wrong, the potions get spilled, etc. And thus is not something easily reproducible. Although gelatinous cubes are clearly deliberate creations.
Many types of monsters are said to have been created by wizards. This is another example of NPCs not following the same rules as PCs. High level NPC wizards have the following special traits: a) able to create monsters and b) completely insane.