Why do Vx and Vy coalesce at an airplane's absolute ceiling?
I think the answer to your question lies in the fact that approaching the absolute ceiling, the terms "climb rate" and "angle of climb", in regards to an airspeed, begin to lose their meanings.

As a plane approaches the absolute ceiling, the climb rate will be zero or close to zero, so the best angle of climb approaches 0 degrees above the horizontal plane as well making your "best-rate" speed also be your "best angle" speed; both the rate of climb and thus the angle of climb being, effectively, zero.

Any speed which yields less than the best rate of climb (if you want to call it that) of zero at the absolute ceiling would be causing a descent, and thus a negative angle of climb as well making any speed other than the best rate of climb speed also not be the best angle of climb speed, which is why that is the altitude at which best-rate and best-angle speeds coalesce.