There are a couple of reasons for this:
* Lapras is a water-type Pokemon: The Rocket Hideout is located in the mountains and doesn't have any water bodies where a Lapras would naturally reside.
* Lapras is a rare Pokemon: While you can catch Lapras in Pokemon Gold, it's not a common encounter and doesn't appear in specific locations like the Rocket Hideout.
Where you can find Lapras in Pokemon Gold:
* Route 43: You have a chance of encountering Lapras in the water here, but it's quite rare.
* Lake of Rage: This is the best place to find Lapras, especially after you've completed the main story.
It's important to note that Lapras isn't a part of the Rocket Hideout story in Pokemon Gold, so it wouldn't make sense for it to appear there.