Why did ‘off-the-shelf' come to mean ‘in stock, ready-made, and easily available’?

Question

I came across the word “off-the-shelf” in the following sentence of Time magazine’s article “The 50 Best Inventions of the Years (Nov.11 issue).

He (inventor of the first synthetic cell) started with ‘off-the-shelf chemical’, and managed to reconstruct the genome of bacterium that successfully ‘booted up.’

I checked out the word “off-the-shelf” with a dictionary at hand, which defines it as “in stock, ready-made, easily available.”

Although this may be a naive question to most of native English speakers, it casted me a question, why does “off-the-shelf” which sounds to me “off the store-shelves” akin to “out-of-stock” on its face come to mean ready made and easily available? Can somebody explain it to me?

Answer 1

Compare to "off the rack" as opposed to "tailored" for clothing.

The implication is that it is a readily available product that can be obtained without a considerable lead time.

Contrast with "bespoke" or "custom made".

Answer 2

I'd say it comes from the idea of something that you can just take off of the store shelf (instead of having to have it made for you), not to something that is already not on the shelf.