Maybe because it is not a good idea. Masking errors is harmful. The information "what you're looking for is not there" is very important, because it lets users identify that something is wrong. Smart redirection can be outright dangerous: what if I am buying a medication, and the smart website silently replaces the correct drug with something similar but wrong? Not to mention the pollution of the indexes of search engines with all the permutations of the same thing they may discover. Lastly, the U in URL stands for unique; the web is designed around unique locators and this rule shouldn't be broken without a very good reason.
Show the user (and the crawlers) a 404, and suggest your corrected URL in the content of.the 404, and let the user know that it's a guess, so they make an informed choice about the situation.