Basically, the reason the question was asked in this text is because they lack a definition of piety. That's what they're trying to do throughout the whole text anyway - find an apt definition of piety. My response above relies on piety's being defined as "actions loved by the gods", which does make the question somewhat redundant, but if we do not have the luxury of defining piety that way, then the question does get a bit more difficult.
One of the main issues here is one coming from translation. When modern society thinks of "piety", the Christian image is often one that comes to mind; a pious person is one who follows God's commandments. Things were not so simple with the Greek gods. However, it is incorrect to argue that there is no action that is universally loved by the gods - for example, to my knowledge, no Greek god has ever encouraged disbelief in the Greek gods. Saying piety is made up by humans is a cop-out answer, since every word and concept, the Greek gods included, was made up by humans.
Basically, this is a trick question in true Socratic style. It seems as if both clauses are true (that is, the gods love piety because it is pious, and what the gods love is pious), but this is circular, meaning the definition of piety cannot simply be "what the gods love". You can read all about this here:
https://en.wikipedia.org/wiki/Euthyphro_dilemmaNOTE: Do NOT think about this as a difficult question that has an answer. This is not the goal of the question. The goal of the question is to challenge a particular definition of piety.