If you're losing 20k exp per week, either get a whm in your LS or get better tanks.
The reason no one is inviting blm's is because, as has been said many times now, black mages are more of a hinderence than a help in level 75 exp parties. The lack of IT-mob camps where a skillchain would be useful means that you'd never MB, which is where you'd really shine. You can get the exp back, you just don't like the multiple avenues available to you, so you're blaming everyone else. Make some friends and party with them, I've partied with less-desired exp jobs before because they were friends, no reason you can't do the same.
Oh, and all the good blm's are off soloing or manaburning or partying with friends already, they already figured out how to get the exp they need.