Why we use XP

There are several reasons why Windows XP is still in use today.

Nostalgia
Millenials had XP during their teenage years and GenZ during their childhood. Some gen alpha even had it as babies. Also baby duck syndrome occur. Anyone past 2005 likely heard of the memes surrounding the start-up sound at the very least. Older users remember it as the new era of computing that started in the newer more security conscious world brought on by the unfortunate events of 2001. Many also remember that paranoia of the so called "Palladium" hardware security which eventually evolved into secure boot on Windows 8 and newer.

Old hardware
Windows XP tends to perform better on older hardware due to it being designed for time-relevant hardware - making it useful for these machines and the ULC PCs from around 2008 such as netbooks, where heavier / "more modern" Linux distributions would not be sufficient (for example: many distributions have dropped support for 32-bit, newer versions of Xorg don't support older (nVidia) graphics drivers, etc).

Legacy Usage
A lot of older software maxes out support at Windows XP. While this could simply be a hint that it was badly written as Vista made many major changes at the kernel level to improve the general security of the system at the cost of overall performance... it's also a deal breaker for many that had software written around ancient Internet Explorer versions, for instance, which can't even be easily emulated in Linux.

Virtual Machines
As highlighted, XP is less hardware-taxing, yet supports enough to get by - so it's actually a relatively ideal Windows OS to run in a virtual machine for whatever task offline. Do bear in mind that without a valid licence for the version you are running in it, you are technically still violating the EULA regardless of the 'obsolete' status.

Embedded/POS
ATMs, Point of Sale Devices, and the like still use XP. Tesco is a good example of this, and a lot of it has to do with the 'building-around-IE6' mistake made in the early 2000s and the ease of deploying it on an enterprise scale on cheap hardware. It costs a hell of a lot to update hardware across a massive company unfortunately.

Because we can
Because we want a break from Windows 10 and its problems. Getting new hardware to work on XP is a challenge. Overclockers use XP because of it's low resources allowing processors to work on benchmarks and gigahertz. Windows 10 reboots for updates all the time yet Windows XP no longer has a working update service.