Why would any people devise a language where “bass” and “base” are sometimes pronounced the same, but sometimes not, and each spelling can represent several words with distinct meanings?
It’s important to remember that the Semitic writing system was not “devised” per se; like all other written languages it evolved from more primitive symbols. It isn’t as if any one person or committee simply sat down and said, “Let us invent a way to write things down, where individual symbols will stand for individual sounds.” It came about as a slow process, from idea-symbols very gradually being adapted, eroded and built into an alphabet, and even more slowly evolving syntactical and grammatical structures. One influence may well have been the fact that fewer symbols to be written (particularly in a time when “writing” was very manually laborious) made the writing of information go faster, with less labor.
(I’ll second the recommendation of the Deutscher book made by Exapno. The book is a fascinating and well-written introduction to the history of language.)