In addition to the proportion of interest decreasing over time as the principal is paid down, there's an additional factor: the mortgage payment is fixed for the full term of the loan at purchase time. So suppose you take out a 30 year mortgage with a monthly payment of $2000 in 2021 and you never pre-pay or refinance. In 2050, you'll still be paying $2000 per month even though rents will have risen with inflation over that time.