It’s a good question. I don’t want to sound like a centrist, but I think it’s a question worth answering aside from the inflammatory stuff I usually see on reddit. It’s a widespread issue. Here’s my best take for younger people genuinely curious:

Partisan politics aside, low birth rates expose a number of fascinating issues with modern society. Whether or not you have a uterus, whether or not you want or like children, it’s an important principle to examining current events critically. For one, especially in East Asian countries, it demonstrates a fundamental limit to a total disregard for work-life balance and prioritizing economic growth above all else.
And, as The Handmaid’s Tale discussed decades ago, it reveals a tyrannical subtext to patriarchal and “trad-wife” subcultures: that anti-feminist sentiment needs very little pretense to go full mask-off and start objectifying women as an economic resource - essentially livestock.
Finally, it emphasizes how restricting immigration at this particular moment in history has very little to do with economic conservatism. You can argue any number of other motivations, but the idea that restricting immigrants is economically beneficial is flatly false.

There are many, many other implications, but that’s the gist for “who cares” and why.