Spend more time on what, exactly? Writing poetry actively, all day, every day? That’s a lot of writing.

How often do people become very good at something without spending the vast majority of their time on it for a long time?

I do not think you are aware that most people's process of writing good poetry is quite involved. It's okay not to understand, but be cautious about implying that another person's artistic work could be done in a spare hour here or there between coursework. It is disrespectful to the hard work of the written word.

It can also involve a lot of reading and thinking, also stuff you need time for

That’s basically what Paul Erdos did for math. People who dedicate their lives to a skill are the ones that tend to push humanity forward bit by bit.