I've had this experience as well, though moreso with getting shirts made. When you buy something off the rack, you've got a pretty damn good idea how its going to fit you, and how its going to look. When you go bespoke, you have to hope that what you've communicated to your tailor gets remembered beyond the measurements he's writing down in terms of shape, and that he nails every single detail you specify.
In general, with a good tailor and experience in communicating these things, this is not going to be a problem, but the forum is full of people who need to work with a tailor several times before they get the suit just perfect. for a lot of people, this is just too much risk and too much hassle when you can get something off the rack that's going to meet a lot of your requirements.