Only oil and similar specific splash weapons that say they need to be prepared actually need to be prepared before they're thrown
The Player's Handbook (2000) for dnd-3e has Table 8–4: Miscellaneous Actions, and on it is listed the attack-of-opportunity-provoking full-round action Prepare to throw oil (128) and directs the reader to page 109.1 Page 109 of that text is the Equipment chapter's description of oil, largely unchanged from its appearance in the Player's Handbook for dnd-3.5e.
However, Table 8–2: Actions in Combat, the similar table in the Player's Handbook (2003) for dnd-3.5e, changes that older entry to the more general and, subsequently, confusing Prepare to throw splash weapon (141) and directs the reader to page 158. Page 158 of that text includes a description of the special attack the Throw Splash Weapon yet mentions nothing about preparing splash weapons!
I'd like to think the change was made in good faith, like maybe to allow future authors to design low-powered splash weapons that are like oil or a reviser overcorrected, thinking the original designers had made a mistake when, in fact, they didn't.
In short, oil is prepared with a fuse as a full-round action that provokes attacks of opportunity. Other splash weapons—unless they use similar language—are not. That is, were splash weapons to take a full-round action to prepare in addition to the time needed in their descriptions, that would make oil particularly terrible as it would take two full-round actions to use: one to prepare because of its description and a second because of the table!
If it makes things easier, try reading Prepare to throw splash weapon as Prepare to throw a splash weapon that has a fuse.
1 In many of the game's previous editions weaponized oil was a big deal: classes and, I think, even some alignments were allowed to or forbidden from employing weaponized oil. Omitting a special mention of Prepare to throw oil, I suspect, might've been construed as deviating too much from older editions, eliminating a touchstone players new to the edition yet coming from older editions might wonder about or lament the absence of. This is, of course, despite the game making oil among the last things one should invest in given the newer game's action economy.