There's pretty much no way around Chansey having low defense. This is because it has 5 base defense, so even if you were to have a defense nature, good IVS, and plenty of EVs in defense, it would still suck. (A measly 119 max) Since you can't really do much about it, you're better off putting EVs in special defense or HP since you won't be able to make it good in physical defense. You could also give it the evolite to boost up its defenses, but even that won't make it's physical defense amazing. They do this to keep it somewhat fair. Do you want to battle a pokemon with sky high defenses, tons of HP, plenty of recovering and disrupting moves, and only one weakness?
Evs are made to complement a pokemon's stats. Putting them in a severely lacking stat won't help them overcome that weakness in most cases. Infernape has low defenses, so low that There's no point in wasting time in putting EVs in his defenses. You might as well go all out offensively.