It's for the same reason that English has silent letters: adding and dropping sounds happens naturally in all languages. It usually happens in rapid speech and helps with pronunciation (read more at Wikipedia
here).
Since the language has such a long written history, the letters are usually preserved in writing long after they've fallen out of the spoken language. Part of it is to preserve comprehension, I think (eemajin if evreewun sudinlee seest yoozing inglish properlee and started too rayt funetiklee!).
Sometimes, the spelling of the word does change to fit the spoken language (e.g.,
bar [']naashaa can be spelled ?? ???? or ?? ???). This process is usually slow and happens over time. Some English words show this too: e.g., "night" and "through" are spelled "nite" and "thru" in some practices.
Also, compare the pronunciation of different dialects: for "three," some say
tlaathaa (as it's written: ????) while others just say
tlaa. Virtually every Eastern dialect says "one" like
xaa, though it's written as
xadh (??) and you can still hear the
daalath in words like
xad`sar ("eleven," literally "
oneteen"),
xodhaa ("al
one"),
xuyyaadhaa ("
unity," where
un- means "one" in Latin), and others.