I think it's a time, and a cost thing.
My younger sister is a tattoo artist, and she actually just did my first tattoo on me this spring, and sure enough it's a line doodle type piece on my arm of a cartoonified version of my dog sporting a pair of 3D glasses.
Between set-up, and the actually tattooing, and then clean-up it took probably an hour, and had I not been her brother, she said she probably would have charged in the ballpark of $100-$150.
A full sleeve is something that will take multiple, several hour long sessions, and cost you likely thousands of dollars. You've got to find an artist that you really like and trust, workshop the idea for your masterpiece, and then sit through a few grueling sessions to get it done, accepting that you're going to have to spend a week or two in-between sessions for healing with a work-in-progress tattoo being shown to the world. That doesn't even account for scheduling. Perhaps it takes you longer to heal than expected and you have to push it out, maybe your artist, or you are taking some time for travel, or doctor's appointments, and need to push it out, maybe you missed your artist's big window of open time and now they're super booked up.. I've heard of big pieces sometimes taking years to complete.
With the small doodles if can be a spur of the moment situation. Travelling, you find a cool shop, pop in, and trade a hundo for a memento of the experience etched onto your body. Over time you can collect these and end up with a "build-your-own" sleeve of patchwork flash pieces. From different places, different artists, maybe in different styles. If you end up not happy with it, they're easier to cover or modify than large pieces.
It's like playing an TTRPG campaign made out of stitching together a bunch of modules, vs running one of the big hard cover books.
With that said.. I'm already planning out my next line doodle tattoo to keep my pup company on my fore-arm.