The reason for that is simple Daryl (I used to be a CNC Programmer too). Decimals are numeric representations of fractions, In the metric system, the smallest unit before going to decimal is 1mm which is very small, so when fractioning it, you rarely need to split it into more than 8ths (.125mm)

But our smallest dimension is an inch, so we have to split it into many more pieces. Now, you can say "Just split it into 10ths and 100ths, The problem with that is that 10ths and 100ths are not fractions of an inch. So 1/8 inch will still be .125. Half of that will still be .0625 half of that will still be .03125, and so on