And if you ask people who don't vote why not, very few of them are going to mention the electoral college. I would wager most people who don't vote couldn't even explain what the electoral college is.
Senate seats are elected state-wide, so they largely go the same way as the presidential vote. If you're in a deep-red or deep-blue state (i.e., nearly all of them), your individual vote isn't going to make a difference.
House seats are district-specific, but:
a) the re-election rate of incumbents is over 90%
b) districts are often drawn to lock-in control for a specific party
However, much to the credit of the sibling response, there are all kinds of local and regional races as well as ballot initiatives that are important.
Or if we analyze this from an opportunity cost perspective, IMO voting is always the right choice. Maybe there's an 80% chance your vote "doesn't matter", but the cost is only 15 minutes of your time every 2 years. Isn't the 20% worth the risk? (OK, I am lucky enough to live in a state where voting lines are short. I understand it takes more than 15 mins for some people.)
In most races, there is little doubt (more than 80% odds) as to who will win. And this extends all the way down to the local level. And voters, candidates and political parties all know this.