Come to think of it, why is SUSE considered as a serious replacement for RHEL and not Ubuntu?
There really aren’t rebuilders for Ubuntu in the same way, and Canonical has a very different place in the industry. I think there was a thing a few years ago about Canonical going after some of the distros based on Ubuntu, but I think that was over pointing directly to Canonical’s repos (rather than hosting their own binaries), which is reasonable.
If Canonical were a public company or owned by one and had a multi-billion dollar business being undermined by clones and freeloaders… I think you’d see similar actions.
See for example this security update, the versions back to 20.04 have links to launchpad, the older versions have no link but say "Available with Ubuntu Pro".
SLES and RHEL are somewhat similar, in that they both use the RPM package format, and they both have a 10 year support lifecycle. They are not entirely compatible though. They also make management software that is supposed to be as interoperable as possible with competitor distributions.
Anyways, last month SUSE announced they would be making an actual RHEL fork, so that's the real reason they are joining this new partnership -- it's not related to SLES, but a new distro they are making.
As for Ubuntu -- it is often touted as an alternative to RHEL. However, people don't like the weird ways in which Ubuntu diverges from the rest of the Linux community (Snap, LXD, ufw, etc.), so some attention is diverted towards Debian and RHEL, which are more focused on packaging software that is already proven. Red Hat does their innovation in Fedora, waits a few years to see if it catches on, then implements it in RHEL. This tends to have a greater degree of success than pushing it suddenly on users like Ubuntu does.
Ubuntu is downstream from Debian. They get a lot of packaging for free.
I don't think I know a single package in Ubuntu that is a Debian package. That is what Ubuntu _does_ -- it takes Debian, repackages and tests and integrates it.
I don't think there are any corporate entities reselling a rebuilt Ubuntu
OEL is just rebuilt RHEL.
Ubuntu has live kernel patching also, but with a different technology. And it's called different.
After Oracle acquired the company that developed ksplice and took the technology private, SUSE started to develop kGraft[1] and Red Hat started to develop kpatch[2], and these were eventually merged and upstreamed[3] into Linux.
[1] https://lwn.net/Articles/596854/ [2] https://lwn.net/Articles/597407/ [3] https://lwn.net/Articles/619390/