The challenges in gluing lens elements into lens barrels include:

Precision Requirements: Lenses must be correctly oriented during assembly. Incorrect alignment can destroy the optical performance of the lens.
Tolerances: Lens elements need to be smaller than their metal housings, creating tolerances that can lead to movement if not properly glued.
Capillary Forces: If lens edges are too close, capillary forces can draw glue between the lenses, which is undesirable.
Design Considerations: Lenses should be specifically designed for gluing to ensure proper functionality and durability.
Skilled Labor: The process requires specially trained personnel, increasing time and costs.
These factors contribute to the complexity and expense of producing ruggedized or glued lenses.

See also: Are the lens elements glued into the lens barrels?