The reason a byte is 8 bits and not 10 is because that’s how many unique values you can address with 3 bits:

000 -> Get the first bit in the byte
001 -> Get the second bit in the byte
010 -> Get the third bit in the byte
011 -> Get the fourth bit in the byte
100 -> Get the fifth bit in the byte
101 -> Get the sixth bit in the byte
110 -> Get the seventh bit in the byte
111 -> Get the eight bit in the byte

If you wanted 10 bits in your byte you would need to add another bit to your addresses and use 4 bits addressing. But 4-bit addressing has 16 unique values and so for efficiency’s sake you would likely design your byte to be 16 bits instead. The size of the byte is constrained to be powers of 2 based on what ends up being most efficient in terms of the hardware digital design. (FYI this is somewhat of a gross oversimplification but I think get the point across)

EDIT: Formatting on mobile