Because it wasn't taught to them properly.
As a former trainer for new joiners in my company, I wholeheartedly support this form of "certification". We cannot take anything for granted.
The game definitely need some form of:
1) Newbie training
2) Knowledge Check of the Newbie Training
3) Refresher Course
4) Certification
5) Re-certification (long term)
So I know this is just a game. So let's make it more fun:
1) Newbie training - this already exists with the Hall of the Novice, it's quite fun, but please introduce more mechanics such as Limit Break, AoE concept, Cooldown concepts, avoiding Boss AoE, standing in friendly AoEs.
2) Knowledge Check is a MUST in any form of training - for this game's purpose, perhaps if you cannot complete the Knowledge Check portion (which can be like a solo trial - think Bardam's Mettle 2nd boss to pass the Boss AoE check), then maybe you cannot lose your Sprout status.
3) Refresher Course is needed whenever there are new abilities or new revamps of skills being introduced, or new job being introduced. This is only based on need. For example, let's take the introduction of DRK. Everyone will need to take a Refresher Course, especially healers, to understand that Living Dead is a skill that is critical for healers to understand. Same thing for other jobs towards other jobs. Here's a new DRG cooldown that will place this marker on the screen, stay nearby.
4) Certification when you reach level 70 or 80 is needed. It can be a solo trial again, and perhaps, the bonus for completing it is a Stat Bonus, or EXP Bonus+, or Achievement, or a nice Gear, or discount to apartment housing, or lotto tickets. Something that will give incentives to completing it, but not punish too much, since it's just a game. (FFXI was more harsh that if you didn't defeat Maat solo trial, you cannot increase in level).
5) Re-certification, maybe 3 months down the line, is needed to keep up our skills. The Stat Bonus from the first Certification only lasted 3 months, so it's worn out. Hence, you need to re-certify yourself. Same thing, incentives for completing it, either another Stat Bonus, even logo next to your name, a furniture.