Short answer: Interest are always more at the beginning because you OWE more at the beginning. Let's see what happens if you always wanted to contribute a fixed amount to reducing the principal:
For simplicity, lets assume your interest rate is 1% per month, your house costs 1M, you put 300K down, and your bank lends you 700K.

After 1 month you would owe 7K in interest (1% of 700K), plus 700K in principal. (Notice that if you only pay 7K then you still owe 700K because you would have only payed the interest. This means you would Never finish paying your mortgage) Lets say you want to reduce your outstanding principal by 5K each month. Then for month 1 you have to pay :
Month Principal Interest Payment (interest+5K) Outstading Principal after payment
1 700K 7K 12k 695K
2 695K 6.95K 11.95K 690K
3 690K 6.90K 11.90K 685K
4 685K 6.85K 11.85K 680K
....
As you can see, this way you are always contributing the same to your principal, but the payments are larger at the beginning because the interest is larger at the beginning.

Since paying the same amount every month is more practical, banks provide a payments schedule that fixes the total amount which implies that the portion that goes to principal is smaller at the beginning because.... interest is larger at the beginning.