Until 10 June 2014 it was not possible to register a domain name directly under .uk (such as internet.uk); it was only possible as a third-level domain (such as internet.co.uk).
Perhaps these sites were registered before 2014...
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.
I can also see them keeping .co.uk out of legitimacy instead of switching. Like you can have just .uk nowadays but everyone is so used to the .co that it looks weird and unofficial if it's not there.
IIRC, one of the historical concerns was that some cheeky Brit would register a domain name that ended with the letter F, like buttf.uk. So “co.uk” became the awkward solution.
That's been around a long time too, I worked for an ISP in the 90s and would have customers using it. They charged way more than Nominet at the time as well.
I used to work for a major (right wing) British tabloid a few years back. We had a promotion with an online storefront that used a .co domain.
The amount of calls and emails we got from people struggling to access the website because they decided to add '.uk' at the end because merely '.co' didn't seem right is astronomical.