I will give you three reasons.
I am a generalist more than I am a specialist. I am not the type of person to make comprehensive summaries. I often leave tasks unfinished and resist investing too long into any one thing.
Being forced to select the top three things makes you hone in on what is important.
I want each idea to be interesting or actionable in itself. If there are three ideas it increases the chance of you benefitting from at least one. Perhaps you’ve heard the first and are uninterested by the second… third time lucky.