Because they are novices, and things that more experienced gardeners take for granted are all new to them. There is also more knowledge required than they can learn at once so have to learn things one bit at a time. I often wonder why the novices at the bridge club never seem to get better, despite having turned up at the club for the last 20 years, and do irrational things with playing cards, irrational even by novice standards (such as partner leads a small card, singleton king comes down in dummy, then novice in third seat holding the ace, instead of winning the trick with the ace, plays low!). With novices I periodically give bridge workshops, they seem to take it in at the time, but come the following week it is clear that almost everything I taught them has leaked out of their mind.