Because it used to stand for extended Berkeley Packet Filter and it has since moved far, far beyond just packets. It now hooks into the entire network stack, security, and does observability/tracing for nearly anything and everything in the kernel ("nearly" because some stuff runs when the kernel boots up--before eBPF is loaded--and never again after that).

Because eBPF is no longer just packet filtering? It's now used in loads of hook pionts unrelated to packets or filtering at all.

Technically it was never an acronym - rather an initialism or abbreviation.