One thing I don't understand is why mortgage payments are fixed in nominal terms over the terms of the repayment plan. This has the effect of meaning they are m

One thing I don't understand is why mortgage payments are fixed in nominal terms over the terms of the repayment plan. This has the effect of meaning they are most painful on month 1 and can be almost trivial by month 360 since they've been inflated away. And that's not even taking into account that earnings tend to increase over time even in real terms. It seems there would be a gap in the market for a loan whose payments increase either at or above inflation (perhaps this does indeed exist and I'm simply unaware of it.). Rent doesn't stay flat over a decade time period so why should mortgage payments?