If you leave your job in almost any other industry you can find an equivalent one that offers more or less identical pay / terms and conditions.
Good luck getting BALPA to agree to that. Imagine what it would do to the promotion prospects of first officers if retrenched Captains from other airlines could be hired straight into the left seat to keep training and insurance costs down.
The low cost airlines get away with it because they have expansion, and turnover of pilots. Whilst command opportunities come faster, they may very well have a shortage of copilots who are ready for upgrade when needed and have no option but to employ DECs.
A legacy airline typically has very low pilot turnover and very little expansion. 10-15 years to get the fourth stripe is normal, join at the wrong time and it can go up to 20 in some airlines.