Things like AT-AT's and Sandcrawlers are the exception to the rule... For me, I'd just as soon they didn't make those at all actually, and that they focused on more feasible vehicles like fighters.
While people say they want those other things, it's like you said. Everyone waits for clearance either because they know they can, or becuase they know they're not getting what they're paying for and vehicle prices are so prohibitive they are willing to NOT bite the bullet on them.
For an X-Wing can they make a scale one? Without a doubt... If these other companies can do an F-16 to-scale the X-Wing is smaller than that. And given Hasbro's construction of vehicles it'd be much simpler, fewer parts, lower cost-to-sales overall (especially if more units are produced), repaint possibilities... Price would vary but if Hasbro laid out the plans right like these other companies do, they'd know what they could price the early release of the ship at and still crank out repaints for online exclusives, store exclusives, etc. and they'd greatly reduce the costs by getting the most out of the mold toolings.
I think a lot of the reason the people don't want to pay the price for the product is because Hasbro doesn't deliver a high quality item for the collector. It's been said that vehicles are mostly something collectors demand though... Items like the Gunship and ARC-170 were great toys and still didn't sell though, which I found odd, but it's possible that kids simply just don't get the vehicles anymore, or not in the ways we did when we were young. And if adults are getting them/wanting them made then quality becomes a big issue in what people are willing to buy for specific prices.
21st and BBI pay way more licensing fees than you'd ever imagine. Almost every piece is licensed, and some are quite expensive, and all being paid for by companies a fraction of the size of Hasbro with a smaller buyer market than Hasbro has for Star Wars... Hasbro just has to think like these companies to make profitable to-scale ships that will actually sell for the prices they want because the collector is buying something more than just a toy...
The only unsure footing I see is them stepping into some grey area with the various licensees who actually do high-end collectibles... Otherwise, if 21st can sell 10 paintjobs of a P-47 Thunderbolt for $50 each and they don't sit long and command high dollars on the secondary market, then I'm sure Hasbro can match that kind of quality with sales in their line.
Short of a movie year though, vehicles have slim pickings mostly and Hasbro's had no real reason to do more since people don't want them and they aren't willing to "do better" to make people want them more for that price Hasbro insists on.
We'll never get a scale Sandcrawler or Juggernaught of course, but Y-Wings, X-Wings, TIE Interceptors, and the like are all quite possible at some point... I tend not to hold my breath though of course.