Many organisations have demonstrated that those of most value to an airline are those who have been correctly identified as having the necessary aptitude. But this is expensive for an airline; first you have to run a selection process and then you have to fund a training programme. Much, much cheaper instead for the bean counters to rely upon a queue of pilots who have obtained CPLs and FI ratings and then worked for a few years teaching at PPL level - even though they may not succeed at Jet training, there'll be other aspirants to take their place..... Some of whom might even have paid for their own type ratings - a very good deal for the airline and an extremely poor one for the struggling newcomer.
Airline training, with a very few exceptions, is in a pretty poor state right now. But with more schools requiring aptitude selection, better quality candidates will ultimately emerge. What is perhaps needed is for the airlines to invest far more in their own futures by committing to funding their own cadet training progrmmes for which they themselves must carry the financial risk; however, such risk would be minimal given proper selection in the first place.
Flying instruction should not been seen as a 'necessary' route to the airlines; it is right now though, simply because there is no alternative. Having paid for their own qualifications, 'hours builders' are struggling to repay their costs and build their total time. Hardly surprising then that these folk wouldn't be terribly keen on seeing their only chance taken away by a flood of other FIs entering the arena who won't have had to invest anything like the time, effort and hard cash as the current crop (c.40%) of FIs have had to.