"you never moved the bodies !!"

Wow, good point. I think I would have freaked out if I was working on that project and I saw that!!!

For one thing, the houses are likely all done with a slab foundation. We see in the scene where Steve and Mr. Teague are on the hilltop that there is new development going in and it's clear that these are slab foundations which means little digging. 6 inches at the most. After that you're also in an area of the world with no real winters. This means that frost line considerations likely aren't taken. Even if they were there is no place in California where the frost line is over 10 inches. So between these two it's very likely that no one has ever dug over a couple of feet deep.

What the real oddity is that the Freeling's pool is already dug so deep and no one seen the coffins that are under it. We see during the storm sequence that the pool is dug more than 4 or 5 foot deep but no sign of a coffin. Later in the movie bodies and coffins come up from under the dug area. How deep did they bury these people? That's really where they should have found the coffins.

I posted this hypothesis a few years ago on IMDb, before they took down all their message boards:

The fact that a cemetery was the source of the haunting makes a lot of sense. But like you say, how did the neighbourhood get constructed without anybody encountering a grave site?

When the headstones were removed, the workers must have been told that another company would do the actual exhumation and relocation. They were most likely instructed to catalog the cemetery plots and markers so the second company could match them up later. Of course, we all know there was no second company, but those who removed the headstones did not. Otherwise some of the workers might have questioned the process and word could have leaked out to the police or to the press.

The homes in Cuesta Verde did not have basements. This is a common building practice in southern California (I assume it must be due to the prevalence of earthquakes... ?). But you would think when the city installed the sewer system, at least one buried casket would have been encountered. I wonder if any construction workers ever witnessed strange happenings while on the job.

I am going to suggest there was a lot of shady politics and negotiations between the real estate developer (proven to unscrupulous) and the city government of Cuesta Verde. A large sum of money was probably exchanged under the table to keep things quiet.

Just to add a bit to this conversation about building techniques in southern California:

https://www.latimes.com/home/la-hm-basement-side-20150509-story.html

It seems that it's more a traditional thing than anything else. Mind you, this article was written 33 years after Poltergeist was released, who knows how new these techniques are.