This person has freely put in the effort to provide a solution to a problem you have at work, and you can't even be bothered to click the upvote button?

Also, maybe this is just because I'm just a forgetful idiot, but on numerous occasions upvoting has proven beneficial to me. I'll be searching for the solution to a problem and I find that I've already upvoted a question which relates to my issue - then if there's an answer I've upvoted, that's probably going to be the best one.

Can't upvote on SO without spending ??? hours farming karma. People start answers with "don't have enough reputation to comment, but in response to the other answer [...]". Left the only answer on several questions, probably averaging 0.2 upvotes per answer.

My high priority work questions: mostly no answers, even years later. I wrote a self answer, years later one or zero upvotes.

I ask a question with an example, the only answer "solves" the example and ignores the question, I self answer correctly when I figure it out on my own, years later no upvotes.

Several questions I arrive from Google are closed without an answer, or duplicate to a completely offtopic question. Or it's an XY question where I still need an answer for X but I don't have the secret question Y.

If you're not actively gaming SO for karma, it can be kind of a wasteland for anything but read only.

Oh yeah, if you don't have enough karma to upvote then I totally agree (especially if you've put in the effort). I was assuming that you had upvote privileges.