Translation:Fourteen, fifteen and sixteen are numbers.
Probably for the same reason 40 is now spelled 'forty' instead of 'fourty' in English; in a word: vowel shift.
"Originally it was indeed spelled viertien. It wasn't until the 16th century that veertien became the more commonly accepted spelling. Phonetic pronunciation was probably what caused the spelling to change. If you keep saying "vier tien, vier tien" over and over in rapid succession, the sound of vier quickly shifts to the same as veer."
— translated from: http://www.goeievraag.nl/wetenschap/taal/vraag/324749/veer-veertien-viertien
(a web link to a scholarly source is also cited on this page)
Ever wonder why the word 'light' is spelled the way it is in English? If you hear the Dutch word licht, it's not difficult to recognize where the 'gh' came from. It wasn't always silent. When the printing press came along and standardized spelling became more of a necessity, archaic pronunciations of words such as 'light' were already well-preserved in the generally accepted spelling while in other cases, shifts in pronunciation were reflected in the generally accepted spelling. The Old English spelling for the word 'and,' for example, was 'ond.'
In some dialects of Dutch, zeven is pronounced zeuven, but the generally recognized pronunciation and standardized spelling is 'zeven.'
Veertien, vijftien en zestien zijn getallen. Als de getallen een volgorde in een reeks aanduiden zijn het nummers.