Because it's amazing to develop in and if you are good at development it's not hard to make performant applications. Don't blame the framework for a lack of skill
This is breathtakingly wrong. Performance bottlenecks are notoriously hard to fix in ReactJS. Once you've exhausted memoization, hoisting, even propagating state via message passing, you have to fight tooth and nail to rip a hot path out of ReactJS' grip. It's horrible. And it's the prize you pay for using a framework everybody and their grandmother can learn reading only a 5 minute blog post (the true value proposition here).