Why isn't FCP part of Core Web Vitals?

You could say the bar for LCP already is quite high. You should enable users to see the largest element as soon as possible. A fast FCP is still important towards early user engagement and thus reducing the bounce rate. For example, it is better to have an FCP of 1 second and a LCP of 2.5, than an LCP of 2.3 and an LCP of 2.4.

But Google might have chosen to not make FCP part of Core Web Vitals as the 'is it happening' aspect of use experience is already covered by the LCP metric.