An application I've just been "repairing" recently has a spot where it uses two separate queries to pull two full table sized lists of values, then manually joins them with a loop, and then manually re-orders the joined values into groups selectively ignoring some rows, and then embeds the the whole reordered list in a web page. The page takes around 20 seconds to load. Switching it to use a single properly formatted SQL reduced load times to under a second.
Another legacy app I'm employed to "repair" has one single 'template' for every page on the whole site. Its first ~500 lines conveniently consist of a giant and highly nested if/else clause to set the page variables and inline javascript.
Such things are the result of "IT experts," "Software Managers," and "Product Administrators" who've never done real software/web development in their lives hiring random "programmers" who have history or psychology degrees and think they can program because they made a form in PHP.
It only gets lovelier when eventually somebody realizes it's a huge security risk and hires an outside development firm to "secure" it. (Giant eye roll. If they couldn't vet a programmer, you can bet they're great at vetting security consultants and contract developer shops.) Did you know that randomly moving code into folders named "private" and "public" for a few thousand dollars can solve giant architectural and security issues like ridiculously easy XSS and SQL injection?
I don't know what the deal is, but a huge proportion of people writing code are plain incompetent.
At my last company we fired someone who created huge amounts of work for everyone (he thought he could secure page content and alert messages by using base 64 encoding as a stand in for hashing and encryption, for example) and a few months later he was hired as lead developer by a pretty reputable educational business.
... sigh