The reason is political.
The Grey Vote keeps the Conservatives in power via FPTP.
In order to keep staying in power, the Conservatives funnel more money and freebies to the Grey Vote to the detriment of everybody else (children and working people). This has gotten so bad over the last 20 years that the Grey Vote has the lowest poverty rate of any demographic. Working people are actually poorer than the retired folks now (which is really crazy).
I would LOVE to have a rational conversation about long-term public funding but from historical experience I can tell you:
The vast majority of the over 65s do not want to hear it. They are simply not interested. Their "feelings" are their own truth, and no "facts" will change their views.
I have come to the conclusion that this problem is being driven by a lack of education among the over 65s.
The educated over 65s (yes they do exist), are more than willing to listen to the facts, and wil be more than happy to discuss the situation. They understand funding levels, lifetime contributions, life expectancy, and economic fairness.
The less educated over 65s (these are found in the leave voting areas) will simply complain and call you ageist, a bad person, "we paid in all of our lives", etc.. They are simply not interested, and that is a function of being unable to understand (education), some degree of denial (not my problem types), and straight up bigotry (don't want my tax going to those foreigners sort of thing).
Worth saying:
The more the situation gets ignored due to politics, the worse the snap financial correction will be, primarily because at that point the funding difference (being subsidised by the working folks and public debt) will be substantial.
What you are seeing now in the markets, is a small window into the snap financial correction that I have referred to.