The main difference between custom and premade book covers isn’t quality (most often). It’s that premade covers are made by designers alone, with no information from author on an exact book. Designers take genre specific cliches, tweak them and try to come up with cool designs and cool ideas that could go on a book. If the idea matches someone’s book, the premade gets sold. If it doesn’t, it doesn’t sell.
With Custom covers tho, the process is 100% focused on a particular book, author provides designer with information about it and then designer goes on to make multiple concepts. From there a process of feedback and iterations starts and can take some time. The end result is usually a partnership between designers skills, author wants and market needs (aka what’s selling now and genre cliches).
The cover quality depends on designer mostly, rather than it being premade or custom cover. If the designer has great skills, both their customs and premades will be great most likely. If designer has crappy skills, both covers will be likely crappy. Custom covers do tend to be more elaborate and include more digital painting or 3D work etc. Due to much more time being spent on them. Thus the price differences between custom and premade covers.