Why is overlapping generation model not a main model in macroeconomics? It seems that overlapping generation model has far more realistic assumptions than ordinary Ramsey-Prescott RBC model for use in macroeconomics. Why is Ramsey-Prescott ones, even New Keynesian variants, are more popular than overlapping generation ones? Is this just due to simplicity, or is there any fundamental problem with the consequences of overlapping generation model?
Complexity issues are always a factor in choosing model frameworks.

In addition, my feeling is that historically the impressive possibility brought to light by the OG models was that an economy could be "dynamically inefficient" (=being characterized by over-accumulation of capital). After some empirical studies provided evidence that the main western economies did not appear to over-accumulate capital, there was perhaps an impression that the model, while structurally more realistic, had not "that much to offer" to compensate for its added complexity...

Not so, according to Philipe Weil, who celebrated the model's first 50 years of existence in the paper
Weil, Philippe. 2008. "Overlapping Generations: The First Jubilee." Journal of Economic Perspectives, 22(4): 115-34.  (freely downloadable).

The paper is an enjoyable non-technical read, full of compacted information, knowledge and insights.  A quote (bold my emphasis):


  "Samuelson’s stroke of genius was to construct a model that makes economies in which the first welfare theorem always holds, absent
  externalities and distortions, look like quite a special case. It is
  hard to escape the conclusion that the features of the overlapping
  generations model are the norm, rather than the exception: after all,
  can we seriously argue, once we understand what “new” means, against
  the realism of a model that rests entirely on the assumption that “we
  live in a world where new generations are always coming along?” In
  that respect, it is not the overlapping generations model, with the
  wealth of interesting issues it raises and its rich welfare
  properties, that is a simple toy model, but rather the competing
  workhorse of modern macroeconomics, the Ramsey–Cass–Koopmans model
  that assumes that no “new” generation ever comes along as future
  agents are all part of pre-existing families. Barro’s (1974) famous
  paper on debt neutrality and Weil (1987) make it clear that such a
  model emerges only if parents love their children (or future
  immigrants) enough to leave all of them positive bequests. This
  condition is very restrictive, and therefore the Ramsey model, with
  its long-run interest rate that always exceeds the growth rate, is
  literally quite extraordinary compared to the overlapping generations
  model."


This is the rallying cry -in his paper Weil details many aspects and issues raised by the model, and the insights that it can provide.