I feel like I'm missing something rather obvious.. So the general rule of thumb is for every meter under ground, the finds are another thousand years old.
But how do they get buried? Is it just people build over it? But why would they leave pots, bodies etc with half a wall?
Or does dirt etc just slowly drift over and accumulates over years, and people build over it not knowing that anything was once there?
So how sites get buried, or get to be where they are at all, is referred to as site formation in archaeology. There's a lot to it considering that each site is completely unique, including where it is located in comparison to the surface.
First though, I would really like to know where you heard the 1 meter = 1 thousand years rule of thumb. I'm curious where that originated from and how much weight is put behind it, because it really mischaracterizes the depth-to-age ratio. Namely, because as far as I've ever been taught, none exist as a rule of thumb.
Here's why:
Let's say you are at a music festival out in a field, and you drop a coin out of your pocket. If that coin makes it below the immediate surface of soil, say because heavy rain/beer spillage muddy the soil or it is trampled below enough feet, that coin can join all the other garbage from the festival in becoming a site. Now any number of things can happen to impact how deep that site becomes. Should the field get grown over by forest, layers of organic material will compile to create new loamy soils that will cover and bury the coin for as long as it is forested. If the field is downwind of loose soil, weathering will gradually move soils to cover the coin. If the field sits next to a river, silt and sand will regularly be deposited over-top of the field by flooding events. These are examples of natural processes that would bury the coin and music festival site. BUT, forests and weather systems and rivers are not standardized across nature. They will all deposit soils unevenly and uniquely, making the amount of soil on top of a site in no way instantly dateable.
Man made forces too can bury sites. The best archaeological example is what is called a 'tell' or midden. If the music festival happens every year for hundreds of years, more and more trash will continuously accumulate under foot, creating artificial layers of soil that are made entirely of made-made material. You get this in cities that have been occupied for hundreds, or sometimes thousands, of years. Digging down into these urban spaces, you'll find layers of broken pottery and building material instead of soils. Again though, there is no single measurement of how quickly these middens accumulate. Cities grow and shrink in population and are sometimes abandoned entirely, so there is no way of measuring a constant rate of growth.
To add to the confusion, all of the above conditions can not just bury a site, they can also decrease the amount of topsoil over a site. Let's say you dropped that coin 2,000 year ago. For the first 1,500 years, it was gradually buried by soil accumulating under a forest to a depth of two meters. Then, 500 hundred years ago, the forest had a fire and burned down. With no vegetation to hold down the soil, the area around your site starts to erode downslope to a lower area (or is carried away by newly formed streams, or is blown away by strong winds, you choose), until your coin is now within a half meters of soil. Then in the last twenty years, some future farmer begins to plow the field where your crazy music festival site is located. The plowing mixes up all the dirt within a meter of the surface. Now, when a future archaeologist finds your coin, it's actually on the surface of a plowed field, and not underground at all. of course that doesn't change the fact that it is still 2,000 years old.
ANY number of factors can influence the depth of a site, be they natural or artificial. So using the absolute depth of an object or site below the surface is not a safe way to determine its age.
Instead we use a process that I'll explain in a part 2
/u/Pachacamac gave a good answer why sites turn to ruin, get buried even with people living nearby and gives an example how a site can get buried quickly because of its local climate. The metre a 1000yrs doesn't seem to apply as in the answer she/he gives it was roughly 30cm equating to 13,000 years worth of topsoil at the site they worked at.