Why are my front suspension forks so stiff?

At 52kg you'd probably have to adjust the pre-load for the fork since it is most certainly adjusted for a higher rider weight. If the fork is set up for a standard male rider, it will more likely be adjusted to 75-80kg. Which would explain the low travel that you experience.

It doesn't sound like you've adjusted the preload. As sold they're probably set up for someone heavier than you. According to the specifications they are adjustable.

However the forks shouldn't reach the end of their travel except on the hardest hits, in fact I'd say that with forks like those (or mine) if they're bottoming out, they're probably not suited to the riding conditions.

Weighing 52 kg, you are at the lightest end of what might be expected for an adult rider, so you will need the spring to be tuned accordingly. A coil spring has a narrower range of adjustment than an air spring, so if you are too light for the spring, it will not compress enough to be very useful. A lighter spring would be needed.

Reading the generic SR Suntour service manual, it states that your fork is shipped with a medium spring and, aside from tuning the preload:

Two additional spring hardnesses are available for SR SUNTOUR suspension forks - softer and harder than the standard coil spring.

There are three springs available then, to suit riders of different weights, at 52 kg you are too light for the standard spring and would be better served by having a softer spring installed. There are customer service numbers in the service manual too, they could guide you in sourcing a spring as they will be fairly uncommon.

It wouldn't be too difficult for your local bike shop to do the work once the part is sourced, however this raises the question of value. How well will the amended fork perform for the money, and for how long? You might consider putting the money (parts and labour) towards saving for an upgraded fork with an air spring which can be tuned much more precisely and easily; the service manual does give the specs for their air forks for rider weights under 55 kg, so this should not be difficult.

Yes, there is sag also for spring/coil forks. Regarding preload, the manual says:

The fork can be adjusted to the rider's weight and preferred riding style via the spring preload. It is not the coil spring hardness that is set, but the spring preload. This reduces the “SAG” of the fork when the rider sits down. A medium hardness spring is used as standard setting. Turn the preload adjust knob clockwise to increase the spring preload and turn it counter-clockwise to reduce it

Regarding your weight: I think you have actually an advantage: usually the cheap forks are too soft. On top of that, I do not expect you being able to compress a fork its 100mm of travel. It should happen on very rare instances. If you did, then the fork would be too soft.

Anyhow, if the bike is used, check that the fork is not filled with water (it happens when left outside): turn it upside down, then try to squeeze it hard. Maybe you will see some water coming out of some ports.

Finally, before thinking of changing the spring of this fork, ride the bike as it is for a couple of months. Then you can (re)consider if you are fine with it, or if you want to upgrade the fork (an option I would recommend).