h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The reason why drystone walls were so popular in days gone by is that they were often (usually) built in places that were very inaccessibly. That meant that carrying raw materials in, including mortar, etc., was not really feasible.
You would not want to push a wheelbarrow of cement/mortar 600 feet up a hillside just to build a 20 foot stretch of wall!

Also, drystone walls could be much more easily repaired by, say, the shepherd who first noticed damage, and without needing to return to civilisation to gather materials.

As a Dalesman, and as a builder, I can confidently reveal the truth.
Drystone walls were built without mortar there was absolutely no good reason to use it!
Water is not widely available on many sites, nor was sand available, nor cement, lime mortar DID exist but was expensive. So, none of the three main constituents of mortar were available within several day's travel(Yorkshire is nowhere near as rainy as the original poster seems to imagine-in limestone country there is often no surface water, no streams or rivers.)

But in this case, that didn't matter, because it was un-needed.
As drystone walls are built to delineate boundaries, it was not a problem, rather a benefit that they allowed the wind to filter through. That's probably why they stand for hundreds of years with so little attention.
The walls are ancient, many date back to the delineation of the ancient boundaries of lands owned by the abbeys and monasteries. In modern times two men can build two metres of wall per day. This does not include collecting, hauling and preparing of stone.

Digging up this threadafter having seen the entry on drystone walls, I thought I'd also add a commenton what I was told when I grew up why people made these drystone walls.

An overabundance of rocks and stones in the fields that needed to be cleared away before the farmer could start using the field for cultivation and sowing crops.
And instead of carrying these big lumps of rock away for miles, they just used them to build walls to designate where one field ended and the next one started, and to keep animals away from eating the crops.