Think of it like this. Say you have 10 shots with a basketball and in order to pass you have to get all 10 shots in. Now imagine you have 5 shots and you have to get the 5 shots in to pass one stage and then 5 shots again to pass stage 2. You have to get 10 shots in for both to pass but the two perfect 5 shots are easier. It's the same with chip yield.
A single large chip has poorer yield than two small chips. Any small defect on the single large chip renders the whole chip useless (unless you sell it somewhere else with cores or GPUs disabled like AMD is alrrady doing with bad console APUs). The likelyhood of a defect on a smaller chip is lower and a defect renders less of your silicone wafer as useless. You can get two non-defective chips and put them together.
Sure, but then why doesn't the PS5 just contain 4 PS4-size APUs? It's not that simple, there are advantages to having just the one APU.