Overall though I think you're thinking about it correctly.
Bottom line is that transition metal hydroxides as a class are not particularly stable. Some are more stable than others, of course, but in general the dehydration reaction is highly favored to produce the metal oxide plus water in many cases. "Because of thermodynamics" is as good an answer as any - trends in the properties of transition metals and the compounds they form are often difficult to tease out in any systematic way, likely because the metal d orbitals are numerous, closely spaced, unevenly filled, and quite sensitive to other nearby overlapping orbitals. Spin states and, in some cases further down the table, relativistic effects, also heavily influence physical properties and reactivity. I guess we could hand-wave explanations in isolated cases but such explanations may have limited value. In this case, as I understand, the tendency of mercury (I) ions to dimerize/polymerize is not well-understood, although relativistic stabilization effects have been vaguely implicated.