The same reason they do everything else they do. They can. Before the halving I was fairly certain most all personal miners would have found a different pool to mine with because they lost exactly half of their revenue. I mean sure everyone did but the pools that pay tx to miners offset that halving at least somewhat. I believe most people mining here have no idea exactly how much they get per block and think, look we did 25 blocks today and I am waiting on all this BTC. If they realized they could be making much more per block at a smaller pool they'd be fools to stay here. But we are talking about people.