Mr. Chan taught in public school for 12 years — sophomores, juniors, and seniors. He would tell every student the same thing: you need to use your time in class to create memorable, impressive moments so that when a teacher sits down to write your letter, they have evidence to draw from.
The longest letter Mr. Chan ever wrote was three pages, single-spaced. That student had given him years of material to draw from. They are still in contact more than 15 years later. Compare that to a senior-year student who asks a teacher they met in August for a letter due in November. There are simply not enough interactions to write something substantial.