I've seen plenty of sellers regularly miss on their KPIs, but still reach or exceed quota, so why have more than 1 KPI (quota attainment)?

Quota attainment & revenue will always the most important outcomes of any sales organization. However, the most mature sales organizations can align on a set of KPIs that will serve as leading indicators to success. 

It’s not to say that consistency hitting KPIs will guarantee achievement of revenue goals 100% of the time, and similarly it’s not to say you cannot meet or exceed revenue goals if you miss KPIs. Sometime all it takes is you being in the right place at the right time with one lucky well-timed deal to hit your quota.

However, if your goal is to be a consistent Top Performer across a longer time horizon (years, decades, your career), well constructed KPIs should serve as a benchmark and enable you/your teams to take more complex goals, like revenue (which relies on a series of coordinated outcomes to achieve) and breaks them into the repeatable inputs that are in your control. 

By clearly defining & measuring the behaviors, frequencies, and competencies that combined will result in success, and controlling those aspects you can control, then on average you will be able to be able to predictably and repeatedly out perform your targets.