It's pretty simple:

Because a lot of those comments will be filled with abuse, and some will be garbage not worth reading.

After a certain point, you get tired of opening the inbox and sorting through it.

It makes more sense if it's something like email, where you likely know most of it is junk mail advertisements.

What makes you think there isn't just as much junk in your comment replies inbox? Not advertising, but just overall junk.

You'd stop checking your email, too, if there wasn't a spam filter. Well, comment replies don't have a filter for quality or tone or politeness. People get tired of reading trash.

I don't read my inbox. Instead I use the same strategy I developed long ago on the forums of old: I check in later to see the responses to certain comments. After the karma system has hopefully moved the shitty or worthless ones down, and I only check the comments where I genuinely care what the responses will be.

But how can people not be curious why they have several unread messages where it's very likely they are responses from humans who specifically responded to things they said?

I've been around the Internet for a long time, just over 30 years now. That curiosity is long dead. I've seen enough and participated in enough discussions to have a fair idea what the replies to most things will be like on the whole. Some of them I'm interested, some of them, meh