Limiting supply?
Perhaps, for some of these guys it is to help create a buzz. You get a buzz for a line of beer people will tend to check out your other beers. Some times I do think things are limited by choice and logistics that they don't want to overcome. An example is many of the Russian Imperial Stouts out there. Some folks have a year round offering of an imperial stout, others claim they can only do it once a year. Truth to that? It could be anyone's guess but to be honest many of the special beers, the Russian Imperial Stouts, Barley Wines, Double IPAs do cost a fortune to make. While a DIPA may not sit as long as Stouts or Barley Wines economics and space dictate a lot of things. If you have a tank or vessel or just plain space dedicated to aging or settling a beer that is costing you money until it is out the door. While I would think that is not the end all be all with beer that is surely something to consider. Add to this if it is a barrel aged of anything. All of these factors drive up the price to the brewer as well as everyone else down the line.
By anyone's statement, these rare beers are also rare because of the added time, money and space invested in to them. To make them special by very definition you are doing something different with them. If you do what you do for them all of the time they cease to become special. I think this is the critical bit with most of these breweries, the things are limited because they must necessarily be so limited.
Surley is local to me and they have various brews that are all time, seasonal or limited release. Darkness is one of those that gathers the most attention, but would I state that it is the best RIS ever? No. I enjoy it because it is local but I can think of a couple other RIS that are around all the time that are better. Some people enjoy the whole festival that goes with it. That adds to the fun and taste and all of it. (Yes I will go again this year because I met a lot of people last year and had a great time. Just remember kids, Minnesota does not allow liquor sales on Sundays outside of bars.)
In short are things done on purpose? Probably so but due to economics more than anything. If I can have equipment and space occupied for six total months to make one beer that will gain me 50k or in the same time and space four beers that each gain me 50k I will go for the four beers. Simplified but I would wager a guess that is what a lot of these companies do.