In the current quarter (1st April 2019 to today), according to the Users page, the voting statistics is like this (this excludes those who've cast 10 votes or less):
In this same duration (excluding today) there have been 300 posts i.e. 300 questions and answers (excluding deleted posts). So it seems most of our users cast their votes on less than 5% of all posts! Similarly, the close vote rate has also dropped drastically. Only 1 question was pushed to the Close Votes review queue in the last 30 days i.e. only 1 user cast a non-moderator close vote in the last 30 days (this data is only available to admins).
If I recall correctly (and judging from the feed in chat), there have been at around 10 off-topic/broad/low-quality questions which I had to close single-handedly, in the past 1 month. This isn't a good sign. Moderators are not supposed to be the only ones doing the review tasks!
Now let's get to the burning question: why don't you vote often? The generic answers supposedly are:
"I'm just a casual reader and I don't find the majority of the questions/answers interesting enough to vote on."
"I avoid voting on questions/answers which I don't understand or do not have the time to read through."
"My bar for voting is high; I feel it's unnecessary to vote unless the post is exemplary or outstanding."
or some combination of the above. I believe these are not really good reasons. You see, ours is still a beta site and for it to grow and graduate we not only need to create content but also assess it and show some form of appreciation to those who take their time to write fairly great questions and answers here. As for point 2, I'd suggest that you start voting even for questions/answers which are well-written, even if you do not have the time to fully read and understand them; the pros far outweigh the cons. Well-written questions and answers (especially answers) take considerable time and effort to produce, and receiving almost nothing in return can be truly discouraging.
I'm not saying that you should only upvote; downvotes and close votes are also as important, in order to weed out the bad content. Stack Exchange runs on the philosophy that good answers are voted and rise to the top and low-quality content is pushed down. So if we're not voting, we as a community are failing that goal! I strongly recommend reading KRyan's answer on this topic.
Moreover, note that the more you vote the more Google and other search engines will pick up our posts i.e. Google specifically optimizes for those Stack Exchange pages which have a large number of votes and views (as they're indicative of the general quality of content on the site) cf. Why is voting important?. Furthermore, a higher vote count on average helps to push questions to the Hot Network Questions (HNQ) list more often, making a site more visible network-wide. And greater visibility implies we'll have a higher chance of being noticed by professionals and academicians in this field. That is really important for our long-term sustainability. We're still pretty low on the number of subject matter experts and that's kinda concerning.
All in all, please start voting more often! If you want to discuss something regarding this or share your opinions and views, you're welcome to do so in form of comments or answers.