I grew up with four languages around me and am currently learning another, having also added a couple of others before. Here's my take on it:

- For a child, every lesson in everything is a language lesson. Everyone you come across will correct you. Everyone will add to your vocabulary. Say you're cooking, or going to the zoo. You'll learn what pots and pans are, all the animals in the zoo, some verbs that are relevant.

- English is very relaxed about its pronunciation. If you're British, "scarf" and "giraffe" rhyme. In American English, they don't. Yet nobody meeting someone from the other side of the pond considers either to be wrong. When English speakers go and learn another language, there's a lot of these close sounds that are wrong. In fact just yesterday in a Mandarin class a guy asked the teacher what the difference between "车" (car/vehicle) and "吃"(to eat) is. He couldn't hear it until we tried a few times.

- English is everyone else's second language because pronunciation is not a big deal. You can often tell if someone is a foreign speaker from their accent, but it's still understandable and nobody complains. By contrast most people in Europe will just swap to English if they hear you say something slightly wrong.

- Institutions are bad at teaching languages because they're not geared towards teaching, they're actually just indices. When you learn something like Linear Algebra or Data Structures, you don't learn stuff in class as much as you learn that they exist. So you go to a DS class and you find out there's a thing called a hash map. You go home, fire up your toolchain, and play with it for a few hours until you get it. Everything you need is out there on the internet somewhere, in English. Same with just about everything else, you learn on your own what's been mentioned in class. If you sit in a language class, you can't do that. Nobody can hear if you are pronouncing things correctly. Also, there's a minimum amount of structure and vocab you need to be able to say anything useful, and you're constantly running into missing ideas, eg "how to I say 'used to' in this language?".

- I suspect the real reason people find it hard is economics. Contrary to this article, I don't think it's actually particularly valuable. This is why immersion doesn't happen. Kids have low opportunity cost, this is why they are able to spend their youth learning a bunch of things. If you move to a new country as an adult, chances are your livelihood depends on some specialist skill that is supported by English, and the rest of the time you're taking care of your family. So you're not getting immersion. You also don't get cultural immersion from the language class. I'm not talking about the fact they eat baguettes in France; your knowledge of France as a culture requires you to access umpteen levels more information than people normally do. How many people are going to know what the French think of secularism, or the Dreyfus affair, from just studying French? I don't think I'll be coming across essays about the cultural revolution in my Mandarin class, either. And then add to that cultural items that every local knows, but isn't considered historically/politically important. The most popular Danish song of all time?