Why do we sing songs and not song them? And, after we sang it, why is it sung?
Well, I don't know, to be honest, but that sort of a change of vowel in words that are related to each other is called an ablaut (it's a German word, so you say it AB-lowt).
You occasionally get the same sort of thing happening in English with plurals: goose and geese; mouse and mice; foot and feet.
Woman/women is another example, and in fact it's a double one: the man/men bit of the word changes, but so (invisibly) does the sound of the o.
Some ablauts are just a bit more subtle. In the words telegraph and telegraphy, for instance, both the second e and the a both change sound.
Ablauts are not only an English thing. The idea was first described by the fourth century BC Sanskrit grammarian Pānini. Much later in Europe, in the early 1700s, Lambert ten Kate wrote about them in a book about the similarities between German and Dutch.
German is a language that really enjoys its ablauts, so here, to finish, is the German word for burst in various tenses.
It's splendid stuff for chanting.
Bersten, birst, berstet, barst, geborsten!
Nuts and Bolts: ablaut. This word was coined by Jacob Grimm (yes, the fairy tale man) in 1838. Ab means off in German, and laut means sound.