We always do, whether they work or not is a question and whether the politicians implement them or not is the major question. The US military and State Department literally have sections dedicated to planning such operations, that's their entire job and they (again literally) wrote the book(s) on how and what to do.
We have good reasons to believe these plans were fairly realistic? Who did them, no-politicized civ-ops types who'd been doing this as part of WW3 planning?
Military and civilian agencies who's job it is to prepare and flesh out contingency operations based on current requirements, information and resources. If implemented as planned and supported by political will then yes the plans were likely realistic and achievable. The main problem was the first Gulf War was indecisive by political decisions while the second failed to make any use of such planning with political will failing to make any long term plans or aims for possible outcomes.
The difference in 2003 was what? A refusal to plan? 'Planning' based on talk radio, editorial page ideologues?
Those "in charge" of the 2003 Gulf War literally wrote a "White Paper" detailing how conquering Iraq and turning it into an "American aligned" state would lead to Middle East (and world) peace with about zero actual details on to accomplish this and even less details on anything beyond "taking out Saddam". American internal politics crippled the 'response' after GW1 and this was aggravated after GWII simply because the political leadership had no idea what to do with either Afghanistan or Iraq once both were taken. Couple this with an unwillingness for "one side" to allow "the other side" to gain any real traction in the situation (that internal American politics) and nothing every really got done towards making either a stable nation which the US could withdraw from.
Yes very much "one side's plan" (and I use the term loosely) was essentially based on ideologue's and talk radio "talking points" rather than the actual situation on-the-ground and was followed up by opposition to the "other side's plan" on general principles leading to a dead-lock state of non-action beyond quick-fixes and patch jobs. Long term occupation is something the US does not do well and the 'goal' is always to reach a point where the "government" is stable and friendly enough to allow a major reduction in forces. (Usually we don't go for full withdrawal but aim to gain a "Status of Forces Agreement" for a certain number of troops/bases if it's a possible area of conflict)
In the context of GW1 our allies were not comfortable with the US taking out Saddam and replacing him but WE had to make plans for just that in case it happened by accident or there was an internal coup or any other shift in power. The thinking during the run up to, and the war itself was that Saddam could not likely 'survive' a surrender or loss of the war but we hadn't counted on most of the allied Arab nations, (including Saudi) backing him to stay in power once the war was lost. (Actually we probably DID 'count' on it in some way. As I noted we try to plan for every contingency) Worse the post-GW1 ambiguity and lack of political guidance kept everything in the area in flux as did the shifting political winds which drove different (often conflicting) policies.
Everyone from the top down "knew" when GW1 ended we'd end up being back to "finish the job" within a decade. None of us quite realized how unhinged American post-Cold War politics had become though.
Randy