Firstly it's because when they started making strings they made unbalanced sets, look up the original fender strings, they are insanely tense on the G and D and super loose on the E, the GHS heavey precision flat set is the same, with 30 lbs of difference between E and G. Also they used 0 and 5 digits for the last number on those early sets as well. SO TRADITION.
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
Meanwhile you look at european manufacturers and it's a little different. Pyramid and TI have pretty balanced sets with odd numbers, it seems the stereotype of german/austrian engineering and precision could be applicable here, both sets of flats i've used were well balanced and within 5 lbs. that's it.