Because of the pathetic salaries, there is no reason for people to start instructing. Yes, eventually you can make it pay, but after spending all that money on training to get a CPL or I/R, it isn't much of a reward to be earning stuff all for a few years to build up the requisite experience.
Mind you, with the low level of experience now being acceptable for CPL instruction, this may change.
MOst people get into flying ecause they want to fly big jets, very few start because they want to fly light aircraft. They are seen as a stepping stone and always will be for the majority of CPL students.
Unless instructing pays far better than airline flying, then there is no reason for the majority to stay and even then most would take a pay cut if an airline came knocking.
I'm afraid this is an almost impossible situation, whilst there are always sensible people who realise that airline flying can be the most boring job in the world, most airline wannabees get into flying not for the love of it, but for the perceived "status", so shiny jets and flash uniforms will always win out.
They may change their tune after a few years, but that doesn't help the situation as by then, they are stuck with mortgages and responsibilities and can't afford the pay cut.