I'm not assuming earth is unique. There have been many earth-like planets that have been discovered.
I'm not even assuming humans are unique, given all of space-time.

It is extremely unlikely that there exists intelligent life other than humans at this time (or within the window-function of time required for us to receive a transmission from however many million lightyears).
Like, it is vanishingly small. The insane series of events that has lead to an intelligent species being dominant on a planet is ridiculous, to be honest.
In other words, humans are essentially unique at this point in "observable" time.

It is extremely likely it is a natural phenomena that we don't understand, or even equipment malfunction, misinterpretation, miscalculation etc.
We have discovered unknown signals, then learnt what they are. Humans don't know everything.
We have discovered unknown signals, then realised it was a nearby microwave, or a dodgy connection, or whatever. Humans make mistakes.

The simplest explanation in order to not have to deal with a new research project is probably "aliens". But the simplest explanation is "natural phenomena we don't understand yet"