A couple of reasons, to prevent people from changing things that they said, and to encourage users to be sure of what they post before they send it, to try to prevent "posting regret."
When it comes to questions you can always just add an update... though I know it's sometimes annoying when you notice a typo you made in an answer or comment, and you can't do anything about it. But ultimately some typos don't really matter too much, it happens to all of us.
We appreciate the feedback and things are on the table for consideration. However, the reason we don't allow it now is its potential for abuse. Users trolling/antagonizing, changing it after, making the responses seem unwarranted and worse than they really are, or irrelevant. Ever changing posts would also confuse the community, rendering the opinions and advice useless with the addition or deletion of certain details or words. We encourage everyone to carefully consider what they post before posting it. Typos can be corrected using updates or comments, and we admins often correct typos we catch in questions as well.