honestly i think it's as simple as people like numbers that end in 0 or 5. 40 60 80 100 is easer to say, to see, to remember, and to hear, than 43 56 72 101. Also when sending specs to manufacturers making them it's easier for them to decipher. SO EASE OF USE
45-60-80-105 is all 0s and 5s and for most constructions, that'll be pretty close to balanced tension. (Some might need 45-60-80-110 to be more on target but I digress.) Yet pretty much nobody makes this as a standard set. And while sure, there are sets where all of them are +20 relative to each other like in your example, there are lots of sets that aren't. Regular Slinkys are 50-70-85-105; Super Slinkys are 45-65-80-100; most other sets are similarly not the same from string to string. Those aren't any easier to remember than 45-60-80-105.