Not only has no tax been paid on it, as it is mainly house price inflation, but it's worse than that, because people forget that they never bought their first house with their own money.
Eg, the 80's, buy for £100K, sell 5 years later for £200K. But you never had £100K to start with, only a £5K deposit. You borrowed £95K, and got MIRAS tax relief on the repayments.
So when you sold for £200K, and paid back the mortgage co the £90K or whatever you still owed on the mortgage, you had turned your £5K into £110K, tax free.
So OP is right, it's completely unfair. Having said that, I will still take advantage of it, and it'll be left to my kids. That's because I want my kids to have as good a life as possible, and that's my responsibility. It's not my responsibility to adjust my financial affairs to compensate for an unfair tax system.