Why does everyone hate i5 6400?

It comes down to the person's situation and what they consider 'value' since it can be subjective to some. As uguv suggested the 6500 is 18.5% faster at stock base speeds, it's around 9% faster with turbo boost clocks. That not based on comparing the max turbo but the slightly lower than advertised turbo boost that would apply if all 4 cores are loaded. Keep in mind if the 6400 says 'up to' 3.3ghz that's 1 core fully maxed out. When all 4 cores are fully used turbo boost drops a bit to around 3.1ghz. Same for the 6500, instead of 3.6 it would be around 3.3ghz.

So between 9-18% more performance for 11% more cost (at least going by current u.s. prices). Prices will change value. It also depends on the person's upgrade habits. If it's someone who upgrades every time something new hits the shelf and they're only going to have the cpu for a few months to a year it may not make as much difference. For those who want to keep their cpu for 3, 4, 5yrs before upgrading you're looking at $5/yr and during those years someone may be stuck with a locked cpu they can't do much with to make it faster.

Another option is to buy the 6400 to save $20, try and sell it after a few months to a year and take around a $50 or more loss on it depending what they're able to get out of it. Then turn around and buy a better cpu. It's generally less overall cost to just invest in a better cpu up front. Most people can afford $20 if they save another week or two and that's not going to be a long wait time, not months or anything. Not when it comes to a longer term investment, it's a pc not a meal at a restaurant or basket of groceries, something purchased repeatedly and in short time frames.

Same with a tv, most people keep their tv around for awhile. At least a year or two, often longer. It's not worth pinching $20 over the course of years to end up with a smaller screen or fewer connection options that could easily bite someone down the road. It might seem trivial but if the cheaper tv only has 2 inputs instead of 3 and 18mo later someone needs that 3rd input they can either deal with 2 and put up with the hassle, buy the tv they should have to begin with or buy some sort of adapter that's liable to cost more than the original price difference. Rarely does saving a few dollars now pay off.

In the end if 2.7ghz were all the speed needed, no gains were to be had from higher clock speeds, then the i7's would all be 2.7ghz rather than 4 - 4.5ghz.