My mother owned an alteration shop so I know a little about the quality of the stitch work. Some places use cheaper machines that are limited in ability resulting in poorer quality. When you buy a quality garment, your paying for materials and the quality of the stitch work.
The Juki machines they use are oil cooled with giant reservoirs of oil to keep these industrial machines working consistently. All this equates with quality work by quality seamstresses.
Not all alteration shops are alike. There's a reason why I by Brooks Brothers suits but get alterations here.