Because so many developers think they are unique snowflakes who are better than everyone else, and a union would just drag them down to everyone else's level. Unions are for people who work on assembly lines, right?
Special snowflakes or not, a group of majority young people won't unionize themselves if the benefit mainly goes to old people. Granted, everybody ages, but unions of today may not survive into the days when the young of today become old.
Which definitely points to a truth, obvious as it is disquieting: brutal violence lies ahead.
Seriously. Unions have been pretty violent for well over a century, as labor struggles have circulated the force of shockwaves created by obstinate dysfunction on both sides, but because of this, a collective memory of known quantities had built up.
Now, in a sudden bout of amnesia, certain mistakes are primed to repeat themselves. Others not so much, but how to guess which errors boil over first, and why, without understanding the roots of each problem?
Technology and any related social change will augment outcomes, but not always in a good way. On the one hand technology empowers mobility and learning, on the other hand, eavesdropping and misinformation.