The bank's fee (interest) depends on how much you still owe.
At the start you owe a lot, so the fee is big. Most of your payment goes to the bank's fee, and only a little pays off your home.
But every payment pays off a little. Then you owe less, so next month's fee is a bit smaller. That leaves more of your payment for your home.
It starts slow, then speeds up, like a snowball rolling downhill.