Boxes are (or at least were when I last discussed this with anyone in publishing, about two years ago or so) far more expensive than putting out even a hardcover.
Some publishers are willing to eat the extra cost. Sometimes (like if you're selling an "intro" game) the extra costs might be mitigated by extra sales. Sometimes, if you really want to include things like dice, or have multiple books (like a player's and a DM's), it's the only viable way to go.
And maybe prices have come down in the past few years, for all I know.
But there were definitely solid financial reasons why they fell out of favor for a good long while.
I've heard that before. The question I can't help but wonder is
why? Despite the cost, board games come in boxes - they always have. Those murder mystery games come in boxes. And so, not surprisingly, early RPG's came in a box. The advantage of a box, of course, is that it (hypothetically at least) contains everything you need to play.
Hasbro (and all the other big game companies) have
always sold games in boxes. The "books only" thinking is probably that RPGs are less reliant on a specific board and game pieces, and so they don't justify the added cost of packaging. I suspect it can initially be attributed to increasing rules complexity and page count. It's obviously easier (and cheaper) to publish longer and more complex rules in book format. But a game in that format only feels like a game when people know what it's for, or have someone to teach it to them.
I guess I'm suggesting that there's another issue at play here. Is is possible that the added cost of the box is justified because it makes the game SEEM more accessible to casual and new gamers? In other words, does the cost of putting a starter RPG in a box make sense because it makes the game seem more like, well, a game?
I'm thinking the answer to that very well might be yes.
Look how many people took up playing RPGs during the boxed set days without anyone to introduce them. Then consider how many new gamers you know who've taken up RPGs without being dragged in by someone who already played. Clearly, if you look at it from a business perspective, something changed about how accessible the hobby was. Was it the phaseout of the boxed set? I think it's entirely possible.