Because the Pacific is really, really, really big. And the USN was making do with resources still barely mobilized, and in a secondary theater.

Kimmel had then had to distribute his meager scouting resources to the most likely initial targets. Midway, Wake, Guam, Johnson Island and the Philippines were all exposed forward positions that required or called for aircraft, be it land based or Catalina's.

Meanwhile the U-Boat threat and Neutrality patrols had called many of those assets to the Atlantic, despite Kings dubious thoughts on their usefulness.

And while it seems ridiculous in hindsight, even today the military still is less active on weekends, because sailors are still humans. And patrol flights, training, or too much flying of any kind can get expensive real fast, especially in a military just barely removed from Depression era budget constraints. Sunday the 7th was unique BECAUSE all 3 battle squadrons were in Port resupplying and repair, including a new paint job after regulations changed. Normally at least one was always at sea training, while an additional battle squadron that would have been at sea had been sent to the Atlantic to serve as a deterrent from German surface ships coming too far West.

While continuosly keeping the fleet patrolling at sea was both unsustainable and counter to established strategy. The battleships sucked tons of fuel and the fleet did not have enough oilers to keep them at sea for long, especially when their much shorter ranged destroy escorts were factored in. To say nothing of consumables and differed maintenance. And in the event of war the fleet was not going to sortie immediately. It would concentrate at Pearl, possibly raid the Marshalls, and generally be reactionary to defend the IDL and East until ready to advance.

I also have to point out the major fleet portions at sea. Both active carrier task forces were at sea reinforcing Pacific outposts. Enterprise was close to returning from ferrying Marine pilots to Wake, and Lexington was on the way to Midway to do the same. While Saratoga was headed towards San Diego to pick up her planes after a refit.