Well, we can find all the excuses that we like but in my experience poor instruction is simply down to laziness. "I've done it this way for years, I'm not going to change now" is another excuse for lack of effort. These types of instructor probably haven't done a pre-flight brief for years so will struggle to do so. Anyone whose attended an instructor seminar or two will no doubt be shocked by the inability to brief demonstrated by some so called experienced instructors. I've employed over the years many 1st class young instructors who taught well and were always ready to listen.
On the subject of poor training. I remember well a conversation with the late Ron Campbell whilst standing with some others at a bar. He had recently filled in at a local club as a holiday CFI. He came to fly with a chap for a club currency check. He said the fella he was so appalling that he asked him who had taught him to fly: "you sir" he replied proudly, through the cloud of smoke, "25 years ago". (Ron like many of his generation puffed at their pipes throughout most sorties - quite something in a C150). Ron's point was that it had taught him a salutary lesson about the reality of things.