Because we don’t want to be Catholic. I dont say it that way to be flip or stupid I say it because its the truth. Yes, for most of us its because there are issues of dogma that we can’t agree with (and I really dont want to cherry pick my beliefs, thats why I could never be Roman Catholic) but, like most things in the Anglican Church it really does depend on the person.
Another problem is that there is a huge difference between ritual and belief. Anglo-Catholics will be very Roman Catholic in their Ritual but not always in their belief (and I know that those two things ought to go together but for some people they don’t). My mother for example loves the ritual of a high Anglo-Catholic Mass but she is so Calvinist in the rest of her theology (also I know its really weird but just trust me

).
**For me, I’m anglo-catholic because I can’t be Roman Catholic but I love the Roman Catholic Church and I’m sure there are people like me out there. **
I hope that helps
Becca