Updated: Oct 4, 2020
Because, think of it this way: Everyone wants to have a ton of people to watch their videos, right? You want to make money, or you want to interact with viewers... Whatever. Doing sub4sub is for others who do sub4sub. What I mean by this is: You don't care about those other people you're subbing to, you're just doing it so that they do it, and bam, you both have another sub. Easy. Except that person cares about your channel about as much as you care about theirs: Not at all. Sub4sub doesn't get you any actual viewers. Subscribers are not equal to viewers. You want viewers who will actually watch your videos, and that's what's important. Sub4sub people will probably not watch your channel, just like you probably won't watch theirs.