So, basically you're asking... why don't I accept or reply to a number of people, in general. Oi, you wouldn't believe how many times people ask that. =P
Friend acceptance for me isn't just a "OMG SOMEONE WANTS TO BE MY FRIEND! 8D *accepts*" it's more along the lines of, do I know this person, how well & how much do I know about this person, factor in my personal opinion of them (maybe), and do I consider them a friend overall. And even sometimes, people randomly befriend me simply to gain the "Hey, I'm friends with a mod" deal, and other things. So, I review that heavily before I press the accept... or decline, button. (You also couldn't imagine how badly I want a decide later option!) or the "you're cool *befriends*" type things... those are the kinds of request I tend to reject, random adds don't usually sit well with me. Yes, I know my friends list is huge, but in some way or another, I know all of them, have talked with them before, and consider them a friend of mine. But just because it's big doesn't mean I accept anyone off the block. Sometimes random requests are accepted (or sent to people) because I'd like to meet the person, get to know them more as well, but that's rare, I guess.
As for replies... Visitor messages, to me, isn't a system I have to reply to all the time. First off, I'm a busy guy, don't always have the time to get into a VM chat, and usually when someone leaves a VM, they might want to have a long chat. Secondly, I may just not want to say anything, perhaps don't know how to build off of their last comment, etc. (That's pretty common) Just because I don't reply shouldn't really offend anyone, either. If it's asking for help, generally I'll reply as soon as I can. But yeah, no one should take it offensively like "OMG, He didn't reply, that son of a biscuit >:|" because that's not how I see things. XD
Hope that answers your question.