Probably Murphy's Law! You have to look at it from the other side though...if every bottle that was produced survived, there wouldn't be very many rare bottles. Don't let sunrunner discourage you on digging though. I know there are plenty of good diggings to be done in New England. It may take a little perseverance but there is still some awesome stuff out there to be dug!
I have gotten discouraged by the number of useless junk I pull out. Im not spry so to spend my whole morning digging only to come away with 100 plain jars or ketchups.
Also discouraged by how hard most places been hit. 19th century sites look like moonscapes from the holes, broked and commons lying around everywhere.
My best luck has been a working mill pond which is frequently lowered way down. I find 30s-40s sodas on the pond bottom.
The other good site is a very old, long ago drained mill pond and the stream running through it. Erosion adds new bottles every flood. Trouble is I have to wait for water level to drop naturally. These bottles all must come from one limited deposit as they date very narrowly to the turn of the century. Bunch of cool very local springwater, milks and preprohibition beers plus medicines. The medicines are all commons so far, the village was on the rail line to Boston so they had access to the larger commercial market. I find stuff from NY and Chicago.
I set aside the farm dump and restaurant/bar for now. As an excited newbie I dug like a maniac because I got blood lust from seeing all the glass. After several backbreaking trips with no good stuff, I been using my free time to scout.
My digs in New Hampshire were mostly farm dumps...often started out new at the top and went old at the bottom. Many of these were not dug if the farm was occupied in the 1970's...but often you can get permission to walk the old stone walls and look for dumps.
You are right on. I walk the stone walls toward the steep stream bank or ravine.
I can't do the privey thing. Too much digging. Working in utilities for 30 years, I made a career decision to never again enter a hole but for one last time, hopefully long from now.
I like walking and looking. You'd be amazed what is close to the surface and exposed by a storm. Or beavers. Beavers like to put dams on top of remnants of manmade dams. Sometimes this diverts water onto land with undug bottles. Old mill sites are on virtually every stream around here.