Just after you wake up, for about 30-60 minutes, you're in a state known as sleep inertia. The CDC recommends not doing critical tasks during this period, but that could just be because it affects performance. They do also say that bright light can more quickly restore performance, which a phone screen most certainly is.

So, let's look into it a bit more. Granted, I can't find anything more than a couple psychologists saying this, so take it with a grain of salt, but it seems like it mostly does come down to you priming your brain for distraction, as was initially stated. You have the least amount of built-up fatigue when you wake up, but if you go on the app that is designed to take as much time and attention of yours as possible, then you are giving away your least-fatigued time of the day to social media, before you do anything productive.

The more things you do in a day, the more fatigued your brain gets, and the harder it is to actually get other things done afterward. On top of that, it can also just be a behavioral thing. If you repeatedly get on your phone every time after you wake up, you are telling your brain "waking up = get on phone," and not something like "waking up = get out of bed and brush teeth" or "waking up = get breakfast."

This can build a dependency over time, which then leads you to, as previously mentioned, taking the time you are least mentally fatigued, fatiguing your brain with high-speed flows of information, and only then actually expending the remainder of your energy on everything else you need to do.