Short Answer: The problem is NOT the doors but the humidity level in the bathroom or home is either too high 

( occurring usually in summer months ) or too low ( occurring usually in winter months ).

Expanded Answer: Hairline cracks are unavoidable in wood cabinets, top-of-the-line or not.   

Wood naturally expands and contracts at different rates with changes in humidity.

The cracks will probably be more noticeable in winter than in summer because heat dries everything out. Every species and grade of solid wood behaves this way, although cracks are more noticeable on light-colored or painted cabinets than on dark-colored ones. These joint lines are normal and DO NOT affect the strength of the door's construction.