We truly do not know the root cause. There are plenty of folks with "degraded" endocrine, cardiovascular, and both systems. Most of them do not develop Alzheimer's.

How many of them have their lymphatic and glymphatic systems effectively halted by parking their bodies in a chair or being bedridden?

Even the most unwell person (in the US) is still dragging themselves to the store and work, on average.

Simple, brief movement that we often relieve the elderly of is the pump that actions the lymphatic system.

Being older brings its own additions to the table.