There are two issues in this thread: a) endogamy and b) DNA bias.
a) Endogamy is not peculiar to Ashkenazi Jews. In the past, most folks married within their own village, and marrying a cousin was not uncommon. In my mother's (Jewish) ancestry, one of her maternal aunts married a first cousin; and her father CL had a "double second cousin" CDL: their fathers were first cousins and their mothers also. And CDL had two daughters who married two brothers. One way of marrying "in the family" while avoiding inbreeding.
On my father's side (not Jewish) I also find a brother and a sister of Granny W marrying two McI siblings.
And I remember a friend who was working on a thesis about intermarriages in "local isolates" - I envisioned distant places far from civilisation, but he was working on communes of Brussels which were villages 5, 10, 15 km distant in the 19th century.
b) Bias. In the past there was an interesting helpfile on FTDNA explaining some essentials of statistics: what a cm means, how the amount of shared DNA decreases, the spread of this amount increases with distance to shared ancestor; and there was a table of likelihood of detection according to distance of relationship. From memory, 50 % for 4th cousins. Some will be detected as "5th-distant", some not at all, others as closer than reality. As the true relationship becomes more distant, likelihood of detection decreases, and only the leading end remains. A tiny percentage of an enormous pool, a vast majority of our matches.
If it's any consolation to patrons of this discussion group, the situation is no better on my non-Jewish side.
Nicole Heymans, near Brussels, Belgium