BreakBlock behavior has this code

@OverRide
protected boolean shouldKeepRunning(E entity) {
return entity.level.getGameTime() <= this.breakTime && this.targetBlockPredicate.test(entity, this.pos, entity.level.getBlockState(this.pos)) && !this.stopPredicate.test(entity, this.pos, this.state);
}
Which returns "false" by default, because <= this.breakTime is set to 0, and entity.level.getGameTime() is greater then 0 pretty much all the time.

This causes the entity to instantly stop using the behavior, and not be able to break any blocks.