(I haven't read the thread before commenting, apologies if this means my contribution is a bit "off.")
At one time, income support did used to pay the interest on your mortgage. (The one time I claimed benefits, in 1991, I got half my mortgage interest for three months then the full amount thereafter, for as long as I was on income support.) So YANBU to think it's a reasonable idea that the benefits system should do this, because clearly politicians at some point in the past agreed with you.
About that time, in the early 90's, there were some stories in the newspapers of people with humongous mortgages, £500K at 1991 prices, which is probably equivalent to millions today, and these people were getting their mortgage interest paid. The rules were changed to put limits on, and I think they have been tightened further since.
Looking at if from the other side, it's probably not unreasonable to think that "benefits" are for poor people, and that the definition of poor people should exclude anyone who owns property. Once they've sold the property, and used up any equity that that then counts as savings, they can be categorised as poor by the same criteria as housing benefit claimants. (One of the good things about claiming benefits in the days when mortgage interest was paid is that any savngs used to reduce the mortgage would not count against you when means-testing was applied. By getting rid of help for home-owners, the government closes a loophole that means home-owners can hide their savings from means-testing.)