Because when you look at it you're like '+10 str +16 dmg for 4K gold?? garbo tbh' and ignore the more intangible benefit of being immune to lockdown, debuffs etcGood on him, fuck buying BKB.
I have thrown quite alot of games by not buying BKB... My friends always call me out on it, but I hate buying it for some reason...
Sometimes I'll be six slotted as a carry without BKB and unable to close out the game, then as soon as I buy it and swap out an SnY or something team fights become EZ mode and we can end very quickly
[edit] and even if the have things that go through BKB it often means you have to prioritise them differently if someone has a BKB so it often still helps anyway