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. 

KPIs drive the behavior you expect to see from your Sales team. Quota attainment is typically the primary focus for Sales teams, how you achieve those results can alter the direction of your business.

Quota from New Logos vs Expansion customers, will impact your future revenue streams differently. Selling a core product vs cross-selling additional products can drive customer retention and ultimately impact churn down the road. Landing a monthly contract vs multi-year commitment allows you to invest more confidently for the future.

Identify what's most important for your business and hold your sellers accountable to them.

Revenue alone should not be the only KPI for your Sellers.

If targets are reached you want to ensure that it will stay that way. So pipeline is very important as well. If pipeline is in good health, you are in a good spot. If not you need to ensure that the activity levels are up. Also, you want to be aware of an upcoming gap in the future. Therefore you need indicators alerting you ahead of time.