I think your reply is the correct one and really made me think. I think you can expand this out to two main reasons.

First, emoticons have limits to their usefulness. They were invented to convey information that might in the real world be indicated by facial expression or other physical cues. Thus the original and most common emoticon, the smiley, indicates what facial expression might accompany a sentence :-)

However, just like in real life, there’s a cost to interpreting the meaning of this language. It is harder to understand a language you don’t know, and it is harder to interpret the meaning of the facial expressions of a stranger, rather than those of your own friends and family. So even with a smiley the true meaning might be suspect, just as the smile of the strange man across the street seems vaguely creepy when directed at you. Bet you never thought of it that way, did you? ;-)

If that last smiley came across as maybe a bit smug or dickish, that might not be how it was intended. If a smiley is used among strangers, the user has incurred a social cost. Even if it is misunderstood by one person, that’s one more person turned off their statement than otherwise might be.

To partially compensate for this, the smiley has expanded out into a huge range of ever more elaborate emoticons, from the thousands of little animated emoji present on some apps and social networks, to the lengthy emoticons made up of obscure unicode that turn up on message boards like (∩ ͡° ͜ʖ ͡°)⊃━*:･ﾟ✧

These may convey finer meaning or they may act as internal jokes among communities, incomprehensible to outsiders. However, it is impossible to entirely eliminate the social cost of using language that may not be properly interpreted.

The second reason is the structure of reddit itself, which exists to create small communities from a larger whole and enforce specific norms within them. The norms are largely set by the moderators and enforced by the users, with the hard work done by the act of voting content up or down. This eliminates most of the cost of what is generally known as “moderation” which on less sophisticated systems such as Facebook gets done by the actions of a relatively small number of individuals physically pressing buttons to delete content contrary to their community’s norms. The moderation cost is immeasurably lower on reddit, thus freeing up time for interested parties to submit interesting content -- the basic act of any community on the internet. Thus, as noted elsewhere here, smaller communities often use emoticons and even have community-specific emoticons. The intent of the emoticons is clear among them, and those that misinterpret must quickly learn better or else risk being driven out by downvoting or worse.

On larger subreddits, especially the default ones, the cost of moderation is much higher because participants may not be aware of norms of a community, or else be uninclined to follow the norms of a community they did not elect to join. Some few, such as r/science, spend massive amounts of effort enforcing their norms (still not as much as might be required without the reddit system, but loads nonetheless.) Some such as r/music have a purpose clear enough that they can get by with a looser set of guidelines. But what of r/pics or r/unexpected? These appear to have adapted to the pressure by concentrating on enforcing very broad rules for top-level content submissions and letting the community members largely police the comments themselves.

The result is a bunch of strangers thrown into a room together and given the broadest subject possible as a topic for expression, like 10,000 people suddenly asked to play Pictionary all at once. As in Pictionary you “win” by drawing, as fast as possible, the picture that will be the easiest to interpret. Hence the distinctive “reddit banter”, a game of rapid one-liners and referential one-upmanship, just like the top voted thread on the comment \¯_(ツ)_/¯

This is interspersed with the biggest emoticons of all -- funny memes and reaction gifs, which often include actual faces and movements conveying information. The social cost of a simple emoticon is strongly in play. When your comment must fight among possibly thousands of other real or potential comments for position and attention, any effort you ask the reader/voter to expend on interpretation is a loss.

The default subs are the engine that drives the larger reddit culture of discourse that permeates all but the most seriously moderated subs, who generally don’t allow emoticons anyway. Thus the general absence of emoticons on the site.

EDIT: They're fn difficult to format right as well, now that it comes to it. Another cost.