>We tick one box.
As do they, when voting for president. Your entire argument presupposes that a voter vulnerable to discouragement cares enough about down-ballot decisions to stay in a line for a long time. It also presupposes that down-ballot decisions are consequential in the voter's state; in a state which is dominated by one party, why stick around?
Canadian polls tend to exhibit very short waits. If anything, US voters should be more prone to discouragement that Canadian voters.
Can we examine whether the presidential box matters more than the others?
Voter Turnout in US Presidential and Midterm Elections.
Absent any more compelling explanations, it looks like the presidential box matters. Common sense suggests there is no unusual phenomenon involving a base of always-voters and entirely complementary tiers of midterm-only voters and presidential-year-only voters; people who vote midterms would surely also vote presidential years.
[Add: we could look at vote totals for different races. I haven't the patience to pull a lot of numbers and add them, so I used Decision Desk HQ and looked at AZ because it is western, it was close, and there was a senate seat up for grabs. 3293430 votes cast for president, 3264042 votes cast for senate. The difference is 29388. While there may be plenty of explanations, and it would be better to compare numbers for more offices across more states, it tentatively suggests there are people who can't be bothered to vote for every office and issue on a ballot. Or, for the tin-hatters, that fraudsters are lazy.]
I conclude that there is a subset of voters who are really only motivated to vote when the presidential choice is on the ballot, and that a person disinterested in turning out for lesser decisions is not going to wait in line if he believes events in earlier time zones have rendered the wait futile.