So, it is possible to overclock your megadrive cpu through means of a popular domestic mod. Why weren't they shipped already overclocked by default or at least given the option for a developer to do it through a special chip or whatever?
Just any given console may not have a chip capable of being overclocked reliably, so its safer to chose a lower clock speed were said reliability is assured.
They could have just as easily chosen to use that ran at high speeds stock, but that would have increased cost.
The chips weren't rated for that high frequency operation, therefore they weren't reliable at that speed. Ones rated for higher speed were more expensive, or required extra heatsinks (which were also expensive).
The Megadrive could also support 128kbyte VRAM too, but shipped with only 64kbyte for cost reasons. If it shipped with 128kbyte, then all the software or external hardware generated graphics in games could've run at up to twice the speed. Including Virtua Racing, all the Mega CD scaling titles, and so on. The reason for this is that those titles are handicapped by not being able to write a full frames worth of graphics into video memory during every frame, but the 128kbyte mode was set up interleaved, so data could be transferred to it twice as fast.
If they used a larger cartridge port then the system also could've used external memory for palettes, allowing 8x16 colour palette banks, for a total of 128 colours on screen. And if they allowed for CPU interrupts on the extension ports, then the Mega CD would have been twice as easy to code for and much faster, OR it wouldn't have needed the extra 68k at all.
It's because, as another anon pointed out, it would be more expensive. I don't know how I enjoyed games before I modded my MD with an overclock.
Every game runs better when the console is overclocked. Whether they need it or not isn't irrelevant.