I don't actually make games, and what little I know about the process I learned here, but I assume that ubiquitous features like resolution support is something that can be "dropped in" from some kind of readily available toolkit, like it's some kind of module or something.
It's not quite that trivial -- don't get me wrong, it's not overly difficult if planned for, but it's distinctly non-trivial -- supporting multiple resolutions means you need to either scale your graphics to fit, have multiple graphics of different sizes or some combination of the two. If you're stretching your graphics will you allow them to stretch to whatever size the player has (potentially leading to problems with the game in years to come), or will you set a theoretical maximum?
You have to decide how to deal with different aspect ratios; should you letter-box wide screen monitors, display more of the playing field, or perhaps show the usual playing field but add an additional graphic that is not visible on a non-wide screen? You could simply stretch your graphics, but you might end up with things being oddly shaped and looking squashed or stretched. If you decide to support multiple options and let the player decide then you've got more code to write, and more different situations that need to be tested.
You have to take into account that your decisions may impact game-play; if your game is multi-player and you've decided to show a larger playing area on wide-screen monitors then you may be giving some players an unfair advantage. Even a single-player game where achievements are awarded or scores are submitted to an online scores-list may lead to complaints of unfairness.
If you're already handling multiple resolutions will you also support multiple monitors (which are becoming increasingly common), and if so how will you do so? If you're not going to support multiple monitors will you provide an option to black-out an unused second or third monitor?
Supporting multiple resolutions certainly isn't the most difficult thing in the world, and I appreciate that your point of view is probably one shared by a lot of players who don't know much about the development process, but it certainly isn't trivial; there is a very real cost in time, effort and potentially money involved in deciding to support multiple resolutions, and it's a decision that may in some cases impact gameplay as well.