Why does people use .co.uk and not just .uk?

To add on to this because the Wikipedia page doesn't actually explain it at all for some reason:

Nominet Ltd was in charge of all .uk top level domain registrations, and they simply decided that they wouldn't allow anybody to register with a raw .uk domain. As far as I can tell, they allowed .co.uk, .org.uk, .me.uk, and other such things according to what the websites claimed purposes were going to be. In 2014 they changed their minds and decided anyone can apply for the raw .uk top level domain, and now newer websites can just be called shitcum.uk

I can imagine a few reasons why huge websites like Google and Amazon don't switch their URLs to google.uk just from a business/corporate perspective. It's probably seen as a lot of money and man-hours to register the new domains, redirect their .co.uk to the new .uk domain (for how long do you even want to pay for both domains?), and the headaches of janky issues arising from the changed domain like possibly third-party APIs breaking or Boomers bookmarks no longer working.