No.
They thought that all fractions could be expressed as combinations of unit fractions.
They and other ancients may have thought that any fraction could be wretten as a combination of unit fractions, but according to wikipedia they (ancient Egyptians) had special symbols for 2/3 and 3/4 and even had a papyrus used by scribes that listed the equivalents for 2/n, so they did not always write fractions as this combination of unit fractions.
Does your question mean a combination of unit fractions, or as a unit fraction?
Because it was their written language.
Egyptians
Ancient Egyptians used a variety of things for a variety of purposes. Ancient Egyptians used dogs, water, tools, chariots, papyrus, precious gems, calendars, and a written form of language called hieroglyphics.
hieroglyphics are a form of written communication that the ancient Egyptians used.
TOBY A.H WILKINSON has written: 'LIVES OF THE ANCIENT EGYPTIANS'
Hieroglyphics.
Karma has written: 'Astrology of the ancient Egyptians' -- subject(s): Astrology, Egyptian Astrology
Contributions of the ancient Egyptians include: Papyrus - the first form of paper Pyramids - an engineering marvel Hieroglyphics - an elaborate written language
For many reasons. The main one being that they could be written on stone.
From as early as 1800 BC, the Egyptians used fractions. There fractions were special because they all had numerator of 1. This is called a unit fraction. So a fraction such as 3/4 was written as 1/2 + 1/4. Fractions the way we use them now were first used in the 17th century in Europe. It is hard to really say who founded fractions since many of the early reference like the one for the Egyptians and Babylonians were fractions but not like the ones we use now.
Evzen Strouhal has written: 'Life of the ancient Egyptians' -- subject(s): Social life and customs, Civilization
John Whittaker has written: 'Studies in Platonism and patristic thought' -- subject(s): Ancient Philosophy, Philosophy, Ancient, Platonists