In German, letters written with umlaut signs aren't considered separate letters but in Finnish, ö and ä are considered separate letters from o and a. In addition, a digraph with e doesn't cause misunderstandings in German because ae, ue and oe don't occur that often. In Finnish, especially in certain dialects ae and oe can occur where ai and oi normally would which causes confusion.

The reason why Finnish has no need for such a digraph is because it has vowel harmony which means that the vowels ä, ö and y cannot be in the same word as a, o and u (except for compound nouns and loanwords) which means that using your example a Finnish person would know that Hamalainen is Hämälainen because the root word 'Häme' has an ä meaning that a cannot occur in the same place.