Firecracker serves an entirely different purpose from Kubernetes. Kubernetes is a cluster scheduler but the containers being scheduled still need a runtime (typically Docker, containerd, or CRI-O) to execute the process within the container. These runtimes typically share a single Linux kernel instance and use the kernel cgroups feature to isolate workloads. In multi-tenant environments where each tenant is running unrestricted code this presents and unacceptably high security risk. If a hacker compromises one container and escapes the cgroup confinement they can potentially impact the security of other containers. Fargate is the building block for an alternative container runtime that uses extremely lightweight VMs and isolated kernel instances instead of the more traditional approach of sharing a kernel between all processes.
More broadly, that is exactly what AWS aims to do with serverless: "De-commoditize" its commoditization by containerization and tightly re-couple your application to AWS. Which is fine, if it suits someone to do that. AWS just likes to use different words to describe it: "Run code without thinking about servers" -- Which is actually only half the story, since you'll still be thinking about all that extra stuff, just that now you'll be thinking about AWS-specific versions of that stuff, in addition to paying the convenience fees.
Forgive my ignorance but this seems like AWS to try and get the lock-in back that they lost with Kubernetes becoming popular