Sounds like a bad ranking system if only 8/32 are playing the US Women’s Am.
Perhaps...but there are other things that may have been done poorly.
The biggest is scheduling the start on the west coast on the day following the Women's British Open. Eight ams played in that event, including the top 2 (Zhang, Lindblad) and #10 McGinty. None is in the U.S. Women's Am field. A shame that top-ranked ams had to choose one over the other.
One could also criticize a qualifying system that reserved only (edit: 25) slots for the top-ranked amateurs, which put the emphasis on local qualifiers. (Edit: But it says something that the top 25 are exempt from qualifying and only 6 of those are in the field)
Which leads into the third reason, which is that only 8 of the top 32 are American. Does that reflect a bad ranking system or a global game where national golf federations invest more in developing top youth than the U.S. does? Seems like it parallels an eyeball glance at the Golfstat college rankings. A number of int'l players may find the cost of a weeklong event in the States to be beyond their means, or they would rather play their own national/regional events--or just enjoy their families for the few weeks they are home from college. (Caitlyn Macnab hasn't had a single event posted in WAGR since the NCAAs.)
Finally, a lot of quality female ams are young--the average age of the field is under 20. So, the need for a parent to accompany minors raises the cost for those traveling internationally and domestically.