It's free in hardwired logic, but the descriptor tables are accessed by microcode. The 286 already had a constant ROM that could hold the FFF8h mask for an ALU AND operation, but no fast shift by multiple bits.

This might be one reason for it, but note that page table entries also use the lower bits for attributes. This means that software can use AND to separate the physical address and other bits, and OR to combine them, with no shift operation required.