www.elitefourum.com/t/gma-card-grading-encapsulation-review-bad-design/13110/1
Here’s a guide I wrote a few years ago about them:) From the outset, the cases are worse than any storage method you can consider using.
To answer your second question, for a long time there were only a few companies grading Pokemon cards on a large scale, which were PSA, Beckett and GMA (and the now defunct MAP grading). While PSA and Beckett weren’t very expensive, usually offering pricing in the range of $4 for large quantities and ~$8 for small-moderate, GMA offered $2 per-card grading cost. And since many of the prominent cards in the hobby were worth between $15 and $50 graded mint, some people considered the cost savings as worthwhile, especially for a quick resale. So in short, there’s lots of them out there, and some people were and still are uninformed about how poor their case & service is.
Also a fun fact… They offer a “grade your own“ program so you can tell them what grade to grade it.