I imagine they’ve just kept the specs from the games made before 1978. Check out the inside of an EM on ipdb and you’ll see they used up all the space.

There could be other reasons too for today if playfield weighs X amount of weight and back box requirements are Y then the dimensions of everything else might be needed to keep the game stable to prevent accidents/ tip overs.

I figure for stability and durability on route, they are commercial units after all and need to be big bulky and heavy like a soda machine/ pool table/ slot machine etc so they cant be pushed around and fall over on a patron or be damaged like EJS said.

It makes a lot of sense, actually. Don't forget these games are meant to make money on route. The coin door is a standard size for many reasons, not the least of which are durability, and accessibility to the cash box inside.

Ok so you have a coin door. Now put that in a wooden frame of sufficient stature to hold it. May as well make it "human scale" while you're at it: bam, there's your front wall dimension. Now extrapolate that to the rear, and there's your box structure. Turns out a flat bottom is far easier to lay down, up, and store (experiments were tried and all failed)... so let's keep that flat from front to back. Oh, the playfield needs a slope? Or a bunch of cool stuff packed onto it? Well now the rear of the cabinet is taller than the front... but at least the bottom is still flat to define a sturdy surface for shipping and storage.

If the cabinet is empty, so be it, but creatives might still fill that space given time and ambition (just look inside a Grand Prix or Black Hole sometime). Newer games might not have the same bulky internal hardware, but the *backboxes* on some have shrunk commensurately. Either way, shipping / storage / durability concerns rule the day. Look up how badly the cabinets for Dungeons & Dragons held up for more reasons why lighter materials and slimmer designs aren't used. And if you've never seen a D&D pin, well there you go...

You should see arcade and redemption game cabinets. A lot of empty space in those. With certain games, you can actually crawl entirely inside without too much trouble.

Take a look at games like dungeons and dragons of blackwater 100. Bally tried to cut some room out of the bottom, wasn't popular. If it's not a rectangle it makes it harder to move and store.

Older games like 60s Wedge heads are probably 3-5" thinner than later games, and had a shorter coin door and longer legs. Not sure why they went to thicker, all the mechanics coils obviously fit. Although I bet it you made a game as thin in the front as a Wedge head but with all the room above the playfield in the back that modern games need, it'd look a bit funny

The comment about the overall weight being a benefit reminds of the story of Harry Williams inventing the tilt. Before that he tried filling the cabinet with sand bags to make the game harder to nudge. He even tried nails sticking out the bottom to prevent people from picking them up.

I think the coin door is the main reason it’s still so big (and tradition). The rest of the cabinet design is based around that and ease of construction. Wouldn’t be surprised if a playfield cabinet dimensions out to a single sheet of plywood

From my semi-understanding of pinball history, Harry Williams figured out pretty quickly the drunks in depression-era Chicago taverns were going to savage his machines to get the balls to go to the desired destination, and invented a tilt bob.
Once the game moved onto the floor with flippers, some bulk likely had to be added to keep them stable and durable enough to still withstand being pounded on and shaken around. That'd be my guess. If they could be smaller, that would have been worked out by now.

Modern stern games are actually pretty light compared to 90s era games.

I almost toppled my GOTG with almost no effort while I was setting it down after moving it with my hand truck.

The backbox is mostly metal and practically empty. Way lighter.

I think the more interesting question is why does the backbox stay so big? It’s just tradition and how the game looks in a lineup

Nice to just use one size leg for all four corners too.

Heighway tried to make games with small/no back boxes and got major push back. People wanted a full size back box, even if it was completely empty

No kidding, a routine double-danger move on a WPC would scoot a modern Stern half a foot across the floor.

I remember the times I moved a TZ, a STTNG (both Superpins) and a Black Hole (still have that one) into my house all by myself, now those games are heavy. If you topple one of those onto yourself when you're setting it up, R.I.P.

Here's an example of a shallow lower cabinet but a large coin door:

https://www.ipdb.org/showpic.pl?id=6778&picno=78707

We have identified such games on the IPDB as having a "drop-down cabinet":

https://www.ipdb.org/search.pl?ft=drop-down.cabinet&sortby=date&searchtype=advanced

On that list, though, not all cabinets are shallow but the front had to be dropped down anyway because of cabinet-front hardware.

I had a Williams Full House with a similar cabinet. Not as pronounced, but similar. https://www.ipdb.org/showpic.pl?id=961&picno=17066

TWSS

In the mid-1960s, Williams tried a coin door that used the type of coin acceptor used in vending machines. Those acceptors are much larger, taking nickels, dimes, and quarters, separating them, and emitting them through different output chutes, all of which requires more height.
.................David Marston