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.

You’d stop checking your email, too, if there wasn’t a spam filter.

Same thing here. People get tired of 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.