I've noticed that I just don't get through games like I used to as a teenager. Used to be that I could pick up a new game and complete the story within a few days easily. These days I'll juggle a few games and I'll be lucky if I finish one of them within a month if at all.

As teenagers we had much more free time.
So, "playing through one within a few days" was no problem.
Today, with RL's obligations in the way...it takes longer.

I'd add to this the (waning) ability to be truly amazed by something. To still get this feeling of wonder. Facing the unknown, not knowing the extent or limits of something.
I remember freaking out as a kid when, after many tries, I managed to unlock the next level of Tetris, because that showed a new background screen I hadn't seen before. It felt like a real discovery and a grand price. Lol.

Yep, what BreOl72 said. My obligations will always take priority, and even after I'm done with those (like for example after I got home from work), I often end up being too tired to play the games I have or I might've still got some energy left but my body automatically went to watching some YouTube videos on the sofa, and end up sleeping in the middle of it and regretting it sometimes.

Energy? I hardly have experience in gaming as a teenager, I only really started gaming in the zeroes, when I was in my thirties. Even then, I took a long time finishing games, but that was because I played a lot of games in parallel. It was not unusual for me to have several installations of Baldur's Gate, each with their own mod setup and with different parties of NPC's, so it could take for over a year for a single campaign to be completed. But now I'm in my fifties, I lack the energy for complex games, even if I do have lots of time (because of my mental disorder - autism - I lack the energy to work more than 5-8 times a week and I can't cope with the stress of paid work at all, those hours mentioned are volunteer's work only). When I was young, of course I had mental problems as well (even if it would took decades more before it was diagnosed as autism), but despite that, I had lots more energy than now. Sometimes I even gamed a whole night trough till dawn breaks!

Through?

Most of the games I play aren't that type.

Over ? under ? in-between ?...

I sort of have the same problem. I have Tourette's/OCD and it consumes a lot of my energy. I only work like 10 hours a week and as fun as games are, when I play longer than a certain amount of time, I feel exhausted and my head starts to feel sore. I have plenty of time in theory but I can't play as much as I'd like to. And yeah, back in my teen/twen days I could play games all night long with only a little nap in between. I miss those days. The irony is that I have so many more games than back then but so much less energy to play them....

Uhhhhhhh...
...What counts as beating SimEarth?

Well, it's still a play-through. As you technically play through it from beginning to end, or to the point you beat it......

For me it depends on type of game, my mood on a given day, how interesting the game is and so on. I finished callisto protocol in one week due to my interest and it's linear simplistic nature. I also find powerslave quite interesting yet I have been playing it on and off for over a month now. Perhaps to savor it longer?

I would add to what g2222 posted. We have lived through the formative era of video games, where with each new generational release the limits of what was considered ground-breaking was broken. New advancements granted wonderment at what was now possible. Going from only knowing handheld Mr.Game and Watch like toys to being boggled by arcade graphics or ps2 era expansive worlds would boggle the mind and inspire endless play time - especially when one is young. Presently, as a cynical adult whose youthful and innocent spirit has been crushed by the weight of the world, having seen what the gaming world has to offer with little improvement, each new release is just another drop in the endless sea of banality. That formally expansive world is now finite, now easily within the realms of comprehension and almost crushingly limiting, like a little fish trapped with a tidal pool.
Even though it's viewing the landscape with nostalgia bias, it feels as though games used to be an exercise in entertainment or artistic construction, or for demonstrating coding expertise. But now the landscape seems soiled by political injections or avarice being the primary motivation to making games.
I also think there is something to be said about availibility. Getting seriously into a game is more difficult when there are thousands of potential candidates out there at my fingertips. When I only owned a single game, it was the best thing in the world.

In short, my ability to finish games hasn't lapsed, but being easily jaded my motivation to do so may be lacking. Sometimes I'd rather just set up and organise everything than actually play...

I remember gaming all night or day long. They were unique and fun times.

Sure, there's scenarios, a time out, or the Exodus...