I don't believe that it is a case of people not wanting to put in any hard work, to keep RP alive. In fact, I have a few choice words I could share on the notion.
To put it simply, there are two words:
Community
and
History
Many roleplayers put a lot of effort into developing their characters. In the course of this, they develop the communities they role play within.
As a result, history is formed.

The history of a role play community is the result of a lot of work.
I wouldn't go as far as to say "Blood, sweat and tears", but the efforts of the people within the community, over the course of time, come together to form history and lore.

Is it becuase people don't want to put in the effort of moving to a new location?
Or, is it that people don't want to cast aside everything they've worked for, over the course of their role playing "career"?
Should the epilogue of a city, or faction's history be "and, they all lived absently ever after"?