The lack of focus is a lure, an oversimplification. It's not just "I can't focus on something I don't like to do". It's more like "halfway into making a cup of coffee, the most important thing in my life became to clean the pantry". Imagine running late, but stopping dead in your tracks with only one sock on, and answering an email. You know it's mad, but you can't help it.
However if you have a job that's lenient enough, you can hide it well. I stopped treatment because I couldn't gather enough data. My job was too easy to know if it worked. It's still the case.
Another thing is that a lot of people learned to cope with it for decades before getting diagnosed. Their life is full of alarms and error-catching mechanisms, so the symptoms are well-managed. If you know your strengths and weaknesses, you pick your battles. It's not an issue unless your battles are picked for you.
It gets named once it becomes an issue in life. It doesn't need to be as bad as being unable to function to get diagnosed.
"Hey John, did you fix that problem with the DB log crash?"
You look up, realize John and the boss are on the other side of the room and you are not part of the discussion.
You put your mind back to that task. Reading the product documentation, "click click, someone across from you taps their keyboard" ... wow, you noticed that 1 hour 30 minutes suddenly passed but you do not recall much of the documentation you just read.
Getting back to it.. a friend messages you a funny joke on whatsapp. 20 minutes of chit chat go by and you realize most of your team already left the office to go home.
There are also other things like fatigue. You'll be able to force yourself to keep up but it takes a toll and eventually you fatigue. Productivity then drops massively and people think you are a slacker.
There’s some breakdown between wanting things and being able to do them. Wanting to do the things but letting down everyone (teachers, friends, family, self). The thing you can’t do might be chores (frustrated that the house is a mess but unable to make it not a mess), keeping up with friends or family (never keeping in touch enough), hobbies (never able to focus long enough to really make progress), work (continuing to function after the initial learning curve wears off), being able to focus long enough to read books you want to read, having conversations with people who prefer to have one conversation at a time instead of four, listening patiently and not interrupting as someone continues explaining the thing you picked up on after their fifth word and they’re on their fifth sentence now…
Everything falls apart slowly and it’s easy to get frustrated with yourself and lose hope. Meds help but being chained to them is a shitty feeling too. They’re a pain to get and require frequent check-ins as stimulants are controlled substances in the USA. The other non-medicine interventions all help in little ways that add up too.