Yes, memory is cheap. But OT maps can easily explode in size.
Theoretical map maximum is around 60,000 x 60,000 x 16 which would be 481 GB in 64-bit pointers to Tile objects alone!
Okay assume we have a smaller map which averages at 2,000 x 2,000 x 14.
That is 56,000,000 tiles * 8-byte pointer...