eMMC comes in BGA packages with many pins (100-200), yet it only uses a few of them (10-30). Why doesn't it come in a package with fewer pins instead, as wouldn't this be cheaper and easier to fit/inspect?
The BGA 153 pinout (designed by committee) can be found in this blog post.
Having all data pins in roughly one place (as opposed to being distributed around the chip) makes it easier to design the PCB as all traces tend to have the same length (and thus the same propagation time) naturally.
BGA packages also have very low pin inductance values, which are desirable for high frequency operation, even if the required number of pins is low.
Once you decide to go for a BGA package (because of the two factors above), you don't really have an option to only make the pads you need for the signals. Uneven pad distribution would lead to reflow defects, poor thermal conductivity and increased mechanical stress during reflow and operation.
The size of the eMMC package is limited by the size of the silicon chip inside the package. Its size can't be reduced (with current available technology).
The number of pins could be reduced, yes, but it would also make it mechanically weaker, because the package would still be fairly large. There needs to be enough pins to keep the package mechanically connected to the PCB.
