Well, the current "thicker card stock" is really just slightly thicker paper./ No comparison to the actual card stock used years ago for both tickets and boarding passes. Originally, the tickets were designed the way that they were because in the era of paper tickets, the right side of the ticket was perforated and designed to be the boarding pass and torn off at the gate as you boarded. The gate agent retained the left part of the ticket and gave the small right part to the passenger. At that time, the tickets were made of real card stock. With the advent of e-tickets, the airline no longer needs the paper ticket to collect its money. Someone obviously figured out that they could change from a true card stock to a slightly thicker paper stock and everything would work the same way. Just guessing here, but I assume that it was cheaper to keep the same format rather than reprogram to use a plain paper boarding pass.