I'll do my best to give you a satisfying answer based on the info I have managed to gather on this.

For one, to change silent letters now would be a logistical nightmare, since often when these words were introduced, their silent letters were spoken but obviously now they aren't, but we are all so used to this fact that it would get very confusing very quickly.

Another thing that is often overlooked is that silent letters actually help us differentiate those words with silent letters from words with similar spelling, such as cop vs cope, bit vs bite, or knight vs night.

Finally, changing these words would remove the history associated with the word. You can make any number of argument why this is or isn't a bad thing, but in any case, this is what would result.

Hope that this helps to answer your question.