It isn't about what I believe. You wrote in ELI5, so my answer assumes you can find more of the things that interest you as long as you get a clear map of the field. I am telling you what doesn't make sense, trying to remove your mistaken thinking.
The reason it is a bad measure is that it doesn't hold up logically, and it doesn't match what we see in surveys.
"A high wage" is meaningless unless you compare it to something. What is "high?"
The most common comparisons are against other members of the economy in which the wage earner operates, or the cost of goods and services for the wage earner. there are a lot of specific numbers you can pull from either of those two sets. In the end, they are both measures of how much opportunity cost the person can suffer before running out of options (or, how much opportunity they have to spend.) Saying someone makes 30k wheras if they live in the US they could make 50k doesn't answer those questions because it doesn't establish a value for 30k OR 50k. Only to a person who has no economic interest in where they live, how they live, and what they are doing, could this be the case, and the is not a legally tenable position (they are required to make choices on those things, and in some regard will have many choice made for them,) and I believe it is an inhuman one.
However, even that doesn't paint a perfect picture. What we also know about happiness and economy is that it has a lot more to do with perceptions of mobility and control, or vestment into the dominant system. So, even if you were financially poor, but actually had a sense of a high level of autonomy and actionable opportunity, you would be, on the whole, happier than the person who had lots of money, but felt they had no control over their circumstances. This is obvious when you think about it, money is not worth anything to most people, it is what we can trade it for that is. If you can already get what you want, then what use is there for money?
Rather than attacking me, I would suggest you do some reading about how nations are compared. What different statistics are useful for, and why some are used and some are not. I'm not in the camp that argues that economics is a science, but it certainly is a collection of methods and principles, and when applied to questions such as "which country is the best to live in," you end up forming very specific questions with very specific and methodological answers.
If your only criteria is income, then you already have your answer. But, if your happiness hinges on more than that (and it almost certainly does,) then you need to handle many more factors. I gave you some arguments that match the mean and the mode of interests from surveys on happiness (check the field "economics of happiness" whose outlook is what I applied to your questions, primarily because it is the field generating the statements that you are seeing, AND that directly addresses the problem you have.)