You said the answer yourself. Parents had genuine love for their children, so they were able to inculcate proper respect in their progeny.
Today’s parents who are so self-absorbed that they can’t even give their kids proper attention, let alone love, should not be surprised to find that their offspring don’t show them the respect the parents feel they deserve. Do today’s parents model respect in the way they act towards their children, spouses, Rebbeim/Rabbonim, and friends? Do today’s parents model respect in the way they act towards their own parents?
You complain that today’s Yeshiva Bochrim are holier-than-thou and disrespectful to their parents, whose rebukes go unheeded. Could it be that the Bochrim are acting the way they learned from their parents? How do the parents react when criticized by others? Do the parents make a point of belittling others? How, for example, do the parents speak about “today’s Rabbonim/Gedolim/Mechanchim?”
Yaakov Avinu waited forty years to rebuke his son because he was wary of the effect it would have. Moshe Rabbeinu learned from him and withheld from criticizing K’lal Yisrael for a similar length of time. How long do today’s parents wait to criticize their children for even the slightest misbehavior? How often do they criticize their children?
There is also the effect of the culture around us, which can not be discounted.
Good parenting does not happen by accident. It takes a lot of patience, time, effort, introspection, planning and deliberation.
It has been noted that the Holocaust erased an entire generation of Torah leadership, and that we are still suffering the effects of this until today. This would explain the rapid deterioration in this area, as in many others.
But of course, blame the Mechanchim. It’s their fault for not teaching and emphasizing Kibud Av va’Eim enough. Well, if your children ever heard you say something like that – even once – there’s your answer. You taught them not to respect those who are supposedly deserving of respect.
The fact is that this, like so many other things, can only be taught in the home. Mechanchim do spend considerable time and effort to teach Kibbud Av va’Eim. However, Mechanchim can support the lessons learned from parental example, but they can not replace such example. They certainly can not overcome a negative example provided by parents.