Because "x, y, and z" are "standard" only in the "the great thing about standards is; there's so many of them to choose from" sense of the word.

Recent example. Client: "We just want an online shop" - great clickety click, Magento is installed, stick their logo in, tweak the stylesheet to match, call it "done".

Over the next few weeks/months: "Where's the blog?" "How do I enter auctions?" "I want to display products with individual pricing, but sell in cases of a dozen." "My SEO consultant says I need some 'landing pages' - where do I make them?" "This doesn't sign people up to out MailChimp email lists!" "It doesn't work right on my wifes iPhone."

Discuss needs with client, install various plugins, a wordpress blog, tweak the templates to make single-pricing/bulk-selling "work" for boxes of 12 (and kick myself knowing it'll bite me in the ass one day doing it this way…)

Then: "How do I make the auction extend the time if someone bids in the last minute?" "How do I enter products that come in boxes of 6 or 8 instead of 12?" "It works on the iPhone now, but not on a friends 2 year old cheapo Android 1.8 phone!"

Then:

"How do I auto calculate freight costs based on both weight and cubic size, for orders with multiple products?"

(at which stage I start explaining combinatorial complexity and the knapsack problem to the client, and run a warm bath and remind myself "down, not across…")