It's representative democracy, like our Congress. People don't vote on bills either, they just vote for representatives and senators. Just like the presidential election, the least populated states get more representatives per person than others. And every state gets exactly two senators, from California to Wyoming, which doesn't seem fair either.
I don't really like it, because it disenfranchises me, but it's intentional and serves a purpose. It was less extreme originally:
https://en.wikipedia.org/wiki/List_of_U.S._states_by_historical_populationRhode Island had about 1/10 as many people as Virginia. But they were both practically sovereign nations, with their own governments, entering an alliance. Virginia got more votes, but Rhode Island (and other small but relatively industrial states) negotiated these rules to ensure a minimum amount of representation for each *state*. And equal representation in the Senate.
That was despite some states having 10% of the population of the biggest (Virginia). Now we have 7 states with populations over 100 million, and 7 states with populations less than 1 million (not counting DC). A citizen in Wyoming has over 500x as much influence over the Senate than a Texan or Californian.
It's part of the ancient issue of states rights. But it's also grown much more extreme with time, despite state rights steadily decreasing. Since the Civil War, which was largely fought over the issue of state sovereignty, the federal government has had the power to fix or adjust this. But like election funding reform, there's not enough political benefit for politicians to take the risk. It would also need to pass the senate, where every state gets 2 votes, and that's not happening.
The District of Columbia is an interesting, rather unfortunate case. It holds the capital, Washington (not to be confused with the state on the other side of the continent). Despite having more residents than Wyoming, it has no representation in the Senate or House (they get one "non-voting delegate", like the non-state territories). Up until 1961 they couldn't even vote for president. Now they get the same 3 electors that tiny states get, the residents instantly going from "unrepresented" to "much more represented than NC, but only in the presidential race".
TL;DR It's dumb and sucks but there's sorta a good reason, it's just gotten WAY out of control but the system makes itself impossible to remove.