I would refine the answer slightly.
A 4x8 is by far the easiest benchwork to construct that a train set will fit on. Any larger (or even smaller) size takes planning, cutting, and additional steps in constructing the benchwork. The train set fits on the 4x8 with what looks like plenty of room for expansion. And there are countless articles on how to get started with a 4x8, and expand it as time goes on. Finally, as you mentioned, the commitment to a given room, $$ for benchwork, and time are about as minimal as you can get.
Fred W
Chip,
I'd have to go with the first two reasons you posted. They have a brand new set-up, need to get it started, and ol' dad was kind enough to build a table with a sheet of plywood on it to get started.
The second reason is just as important. They want to "try out the hobby." They use the 4 x 8 to find out if they: either got what it takes to become a fairly decient modeler, or fail miserably before dumping the big bucks into the hobby.
I think we all started on that first layout on a 4 x 8 sheet of plywood in our youth and as time went on, learned a whole new way to model and carry on to higher levels from there...Maybe it's as simple as: because it's fun....chuck
Chip, they want to play trains, not operate them. They learn how to "run" trains later as they realize there should be more to simply running a loopty-loop for an hour a day.
They realize a train set is achievable for $40-$80, know they need a comfortable and safe place for it, and they all have a couple of saw horses and a sheet of 1/2"ply lined up against the side of the wood shed. Too easy. Card swipe, dash home, Dad's all excited for once, wife feels that he's out of her hair, and they all get a fun interlude for a few nights..................a very few nights.
Then comes the disenchantment, then some reading, then some learning, then some more realization that enjoying model trains, like anything else, takes more than a simple plug and play.
How long does it take for a child to get tired of a R/C car like those that were popular back 12 years ago or so? About a week?
The two most convincing reason I've heard are:
1) They have a train set and they want to get it working fast, so they get a sheet of plywood throw it on a couple saw horses and in two days have something running.
2) They lack the commitment. They want to try model railroading to see if they like it.
But there's always.
3) They heard that a 4 x 8 is the starter layout.
4) They think they don't have room for anything else.
5) That's what the magazines show to get newbies started.
Chip
Building the Rock Ridge Railroad with the slowest construction crew west of the Pecos.