Because writing anything...and "putting it in stone"...so to speak, removes any flexibility for temporarily changing the boats' routes based on guest load demands...hense, fewer guest complaints....haha...maybe....yeah, right!
In any event, it has nothing to do with the sun in the pilots' eyes but poster clkelly has it pretty right, otherwise.
Don't look for the info regarding route reversal to be posted on any signs at the resort docks on Bay Lake.
WDW will build a bridge over the EWP channel off 7 Seas Lagoon and extend the monorail to everywhere before that happens.