I'll change a few words, and hopefully illustrate why "[countries] feel the cons out weight the pros", doesn't look the most likely explanation to me:

"There are many reasons both to have gerrymander and not have gerrymander. The USA has gerrymander, because, one must assume, the USA feels the pros outweigh the cons."

Countries don't make the laws. They delegate that job to politicians. It should come as no surprise that even for an obviously anti-democratic practice like gerrymander, politicians get away with adding laws that favour them personally over democracy.

As for the arguments on www.idea.int, they say the leading one is "the leading argument against compulsory voting is that it is not consistent with the freedom associated with democracy". It's a circular argument. You don't have complete freedom under democracy, and there is no definitive list of freedoms democracy does give you. It varies from country to country. In some countries you don't have the freedom to not vote. So the argument made is really: "the leading argument against compulsory voting is that it is not consistent with the freedom associated with definition of democracy I just made up".

The other argument they raise is "random votes". True, there are random votes - 7.3% according to them. But that 7.3% is swamped by the 40% of additional voters in the USA who vote if you had compulsory voting. Besides, I'm suspicious of that 7.3% figure. In Australia we measure these things as best we can, and the figure is 1% .. 2%. Informal votes are around 5%, which if you add them together does bring the total to 7%. A vote is counted as informal votes when it's impossible to determine the voters intentions. Some are deliberate protests, but it seems most are just mistakes. In any case, they aren't "random votes" and don't effect the outcome directly so the 7.3% figure is misleading.

I've never head an Australian (where we do have compulsory voting) make what www.idea.int says is the leading argument for compulsory voting, which is that "decisions made by democratically elected governments are more legitimate when higher". Again, since I've never seen a solid definition of "legitimately elected government", I suspect it also suffers from the "definition I just made up" problem. I fact I've never heard anyone make any of the arguments on www.idea.int's list.

What Australians do say is it makes detecting many types of voting fraud easier, because it becomes trivially easy to check people votes just once: count the votes. Because detecting voting fraud is easy, there are no stringent ID checks. You just turn up, no ID required, get your name crossed off the electoral roll, get your ballot paper and vote (or not). So it in compulsory effect makes it easier to cast a vote. It also makes voter disenfranchisement much harder for a pollie to pull off, because people start yelling loudly if they cop a fine because someone makes it near impossible. Surprise, surprise, disenfranchisement is common in the USA: https://en.wikipedia.org/wiki/Voter_suppression_in_the_Unite... It's not a thing in Australia.

Those are clear and obvious effects. A debatable one that I think is true is it pulls the vote toward the centre. The people pushing for extreme views always vote because they want things to change. Those that are happy with things as they are sometimes assume it will continue if they do nothing.