Amazon is the one selling all those Chinese-made machines, it seems to me. If you looked elsewhere, you'd find printers made all over the world, even in the USA. Your question could be asked about all sorts of manufactured products, not just 3D printers. There are a lot of advantages that manufacturers in China enjoy, such as low labor rates, cheap access to supplies and raw materials, inexpensive real estate (although that's changing, especially in the big cities) and a lack of all those pesky regulations on their emissions, labor practices, consumer relations and intellectual property violations.

Actually, though, there are lots of companies making 3D printers elsewhere, despite all that. Here are a few made in the USA: https://printrbot.com/ https://www.alephobjects.com/ https://www.seemecnc.com/ https://store.typeamachines.com/