Because megapixels are only one part of camera performance. Expensive flagship bodies like the Canon 1D X II or Nikon D5 are designed for different priorities than high-resolution models.

Their target use is often sports, news, and action work, where speed and reliability matter more than maximum resolution. Lower-resolution sensors produce less data per frame, which helps the camera shoot faster bursts, clear buffers sooner, and move data to memory cards more quickly. Those limits are not solved just by “adding more circuitry,” because sensor readout, memory bandwidth, storage speed, heat, power use, and reliability all matter.

Higher-resolution models are aimed more at subjects like studio, landscape, or fine-art photography, where maximum detail is more important than very high frame rates.

Using a higher-resolution sensor and simply combining pixels later is not equivalent to using larger pixels in the first place. It adds processing overhead, and because of the color filter array and noise behavior, it is not a perfect substitute.

So the more expensive camera is not “worse”; it is optimized for a different job. In flagship pro bodies, you are often paying for speed, durability, autofocus, responsiveness, and dependability rather than the highest megapixel count.