Why would any major take flow through from their own regional? Usually these flow through folks are the most senior, most experienced captains: check airmen, sim instructors, training captains. Now your regional that already has a problem finding qualified people (nobody is going to work in this industry for the abysmal wages that are being offered, bonusses are not the solution) is going to lose its cadre of people that can train and safeguard the bottom of the left and right seat seniority lists? It takes a lot of time to regrow this group of people. So why take it away? Once at the major, they are just a number on the bottom of the seniority list, so why not take them from the competition?
The whole flow through interview guarantee is just a carrot. Doesn't cost much to set up interviews, but it really boosts morale. Typing an email address on top of a "thanks but no thanks" letter takes what - 10 seconds?