OP, I do think your DD has been particularly unlucky, and so sympathise with how you both must be feeling.
It most likely does stem from the fact that the majority of kids that she invited were boys.
IME, birthday parties do generally involve the same kids inviting and being invited (because that's the (wider) group they play with), but it's not a hard and fast rule, and your DD has been particularly unlucky to not get the return invites.
When my DC were 5, 6, 7, parties were generally mixed - boys and girls. But now they're 8, 9, 10, that's usually not the case, plus they tend to be smaller in number now.
For DD's 6th birthday she invited a couple of boys, and didn't get a return invitation back to one, and that Mum and I are good friends. They have two boys, and always have all-boy parties, that's their choice - or more likely their boys' choice. DD invited that boy again for her 7th and she did get a return invitation the following year to his, but was the only girl there. Not all that much fun for her.
You also say that people shouldn't accept invitations for their boys, if they have no intention of returning the invitation because they're going to have an all-boys' party themselves.
Well, that not realistic. If you're kindly invited to a party, you accept, because presumably the birthday child wants your child there.
Imagine being the host mum, and people were declining your child's invitation because they knew they weren't going to invited your child to their party. That's much, much worse, surely.
This year, DD had a smaller group to her 8th and they were all girls.
I do think that you have been unlucky, and that it's for an identifiable reason (inviting mostly boys), and that people are not intentionally excluding your DD, they're just inviting people on the birthday child's list. There is no malice.