Because they aren't really needed anymore.

Old CRT displays suffered from a phenomenon called Screen burn-in. These old screens used a phosphor coating on the “screen” part, which would create light when exposed to the CRT’s electron gun. This coating loses its luminescence over time, so uneven wear could occur if the screen was left to display a static image for too long. Essentially, the image was burned into the phosphor lining.

Screen burn-in could result in a “ghost” of an image appearing on screen, despite a changing image underneath it. In severe cases, this can even be seen when the screen is off.

Image: r/mildlyinteresting - The burn-in on this CRT TV (it’s turned off)

As you can see, the unmoving CCTV cell borders have burned themselves into the screen over the years.

Hence the need for screensavers: on a desktop PC, they'd avoid having the CRT display a static image for extended periods of time.

Burn-in was also a concern for Plasma displays.

However, modern devices use LCD screens instead of bulky, power hungry CRT/plasma displays.

LCDs mostly don't suffer from burn-in, at least not anywhere nearly as severely as older display technologies. They suffer Image persistence instead, which has similar effects but is generally reversable over time.

OLED displays have been shown to be somewhat more succeptible to the effect, as a few smartphone manufacturers have found out over the years.

You can still use screensavers on your PC. However, their main purpose (preserving display health) is mostly irrelevant at this point given an LCD’s low susceptibility to burn-in, especially since most PCs shut off the screen by default anyway instead of using a screensaver (now that laptops are a thing).

As more and more people abandon their PCs for tablets and smartphones, screensavers fade in relevancy even more, as these devices will also shut off their screens outright when not in use.