Learn Italian: Forza, ascoltate l'arbitro! - Come on, listen to the referee!

Because this is a command given to more than one person (the plural "you", or voi). For regular -are verbs like ascoltare (to listen), the voi imperative form is ascoltate, which is exactly the same as the regular present tense.

How would I say this command to just one person?

If you were commanding a single friend or someone you know well (the informal tu), you would use ascolta. The sentence would simply be: Forza, ascolta l'arbitro!

In English it's "listen to the referee." Why is there no word for "to" in the Italian sentence?

In Italian, the verb ascoltare already means "to listen to". It connects directly to the noun without needing a preposition. So, you never say ascoltare a, just ascoltare followed directly by what or who you are listening to.

Doesn't forza normally mean "strength"?

Yes, its literal meaning is "strength" or "force". However, as an exclamation, Italians use Forza! all the time to mean "Come on!", "Let's go!", or to encourage someone to do something.