I’ll tell you why - metal are the most widely available. Also, available are standard white plastic which I’ve found included in pastry decorating sets or kits. I’ve seen professional pastry chefs use polycarbonate tips, but I haven’t seen them for sale to the general public. Plastic is a little more difficult to clean (grease tends to cling to it, but can be washed off with a little elbow grease). Metal can bend, but only if you’re careless. Metal can be sensitive to room temp, which has only been a problem for me when piping ganache - I had put the piping bag down for a while and the ganache firmed up in the tip and was extremely hard to push out (had to heat the tip a bit to get it out).
As far as bags are concerned, I love to use disposable. No matter how many cloth, nylon, or plastic lined pastry bags I’ve purchased over the years, they all get stained and eventually turn rancid. Disposable plastic bags are your friend!!! Parchment is another good disposable option. I frequently use heavy duty freezer storage bags for small piping jobs (stay away from non-heavy duty or non-freezer bags - they bust at the seams under the slightest pressure.
