not keen to take apart our computers and stick new parts in them.
USB gets around having to open up a machine. There have been USB ADSL modems in the past, and web cams, and so forth, and it was brought in to assist people with not having to open a machine up. Whether there are drivers for your OS (Win 7 / XP / etc) depends on the device and age.
Opening a PC has been an increasing rarity over the past 10 years, but I would state it as a fact, that here in the UK (where many MN users live) the odds of getting a desktop PC with a Wi-Fi card (PCI card) are pretty low to infinitessimally low.
You'd have to spend days researching web sites to buy one "off the shelf" at any major or medium retail chain, and doubt many shop staff could point you to one, though PC World just might if it was a Philips (only brand I've seen with it and even then, only on a few models !).
Not even Medion (a German firm) put Wi-Fi in as standard, as far as I know. Maybe Apple do, in case those wanting a budget wi-fi adaptor (off Ebay, for example) will find there's no suitable software, and get narked. Doubt Apple would show just 10 quid for it, if they ever did a breakdown of how they justify the cost of the units some people buy.
Then again, perhaps there's more take up in N America, where thinner internal walls (or larger open plan areas in mid/ higher end homes) make it more likely that wi-fi is used.
However, whatever the case is made for / against wi-fi, the fact is that it is uncommon in the main, in the UK, for any desktop to have it by default, as it has not previously been needed (it was common to have a single PC in a home and that had the modem plugged into it). I can find you a few dozen Ebay listings which may include it but they will be exceptional compared with retail sales.
Laptops are a different matter, the (previous) assumption being they would be used with wi-fi at work and in hotels or conference centres. The growth in domestic use of laptops (and popularity with teens as they can use them for FaceBook, Twitter porn etc in their bedrooms) has increased the demand for wireless routers at home (probably most growth in last 5-7 years) while earlier ADSL routers, and some still around today, had no wireless interface at all.